Smart rollup node#

This page describes the Octez rollup node, the main executable supporting Smart Optimistic Rollups.

The Octez rollup node is used by a rollup operator to deploy a rollup. The rollup node is responsible for making the rollup progress by publishing commitments and by playing refutation games.

Just like the Octez node, the Octez rollup node provides an RPC interface. The services of this interface can be called directly with HTTP requests.

We first cover the operation of the rollup node and the corresponding workflow, using some predefined rollup logic (called kernel), and then we explain how the logic of a rollup can be defined by developing a custom rollup kernel.


To experiment with the commands described in this section, we use the Weeklynet. In this section, we assume that ${OPERATOR_ADDR} is a valid user account on Weeklynet owned by the reader.

Notice that you need a specific development version of Octez to participate to Weeklynet. This version is either available from docker images or can be compiled from sources. Please refer to the Weeklynet website for installation details.

An Octez rollup node needs an Octez node to run. It is recommended that the two nodes run on the same machine. If this is the case, there is no additional configuration required of the Octez node. If they are on different network interfaces, the Octez node needs to allow the rollup node to make specific RPCs. To achieve this, one can add the following to the Octez node configuration file, where <ip.address:port> is the address at which the rollup node can contact the Octez node.

   "rpc": {
     "acl": [
         "address": "<ip.address:port>",
         "blacklist": []


Configuring a public facing Octez node this way exposes it to DoS attacks. However one can allow all RPCs on the Octez node to be accessed locally while still keeping sane defaults for outside accesses by specifying an additional RPC server with, e.g., --rpc-addr --rpc-addr

We assume that an Octez node has been launched locally, typically by issuing:

octez-node config init --data-dir "${ONODE_DIR}" --network "${NETWORK}"
octez-node run --data-dir "${ONODE_DIR}" --network "${NETWORK}" --rpc-addr

in a terminal where ${NETWORK} is of the form and ${ONODE_DIR} is a path for the Octez node store, by default ~/.tezos-node.

The commands will only work when the node is completely bootstrapped, and therefore the current protocol on the target network is activated. This can be checked by:

octez-client bootstrapped
octez-client rpc get /chains/main/blocks/head/protocols

In case you do not already have a user account, you can generate one with:

octez-client gen keys "${ACCOUNT_NAME}"
octez-client show address "${ACCOUNT_NAME}"

Then, the ${OPERATOR_ADDR} can be set to the hash value (tz1...) returned.

Finally, you need to check that your balance is greater than 10,000 tez to make sure that staking is possible. In case your balance is not sufficient, you can get test tokens for the tz1 address from a faucet, after your node gets synchronized with Weeklynet.

octez-client get balance for "${OPERATOR_ADDR}"


Anyone can originate a smart rollup with the following invocation of the Octez client:

octez-client originate smart rollup "${SR_ALIAS}" \
  from "${OPERATOR_ADDR}" \
  of kind wasm_2_0_0 \
  of type bytes \
  with kernel "${KERNEL}" \
  --burn-cap 999

where ${SR_ALIAS} is an alias to memorize the smart rollup address in the client. This alias can be used in any command where a smart rollup address is expected. ${KERNEL} is a hex representation of a WebAssembly bytecode serving as an initial program to boot on. From a WASM bytecode file named kernel.wasm, such representation can be obtained through

xxd -ps -c 0 <kernel.wasm> | tr -d '\n'

To experiment, we propose that you use the value ${KERNEL} defined in the given file.

source # defines shell variable KERNEL

If everything went well, the origination command results in:

This sequence of operations was run:
  Manager signed operations:
    From: tz1fp5ncDmqYwYC568fREYz9iwQTgGQuKZqX
    Fee to the baker: ꜩ0.000357
    Expected counter: 36
    Gas limit: 1000
    Storage limit: 0 bytes
    Balance updates:
      tz1fp5ncDmqYwYC568fREYz9iwQTgGQuKZqX ... -ꜩ0.000357
      payload fees(the block proposer) ....... +ꜩ0.000357
    Revelation of manager public key:
      Contract: tz1fp5ncDmqYwYC568fREYz9iwQTgGQuKZqX
      Key: edpkukxtw4fHmffj4wtZohVKwNwUZvYm6HMog5QMe9EyYK3QwRwBjp
      This revelation was successfully applied
      Consumed gas: 1000
  Manager signed operations:
    From: tz1fp5ncDmqYwYC568fREYz9iwQTgGQuKZqX
    Fee to the baker: ꜩ0.000956
    Expected counter: 37
    Gas limit: 2849
    Storage limit: 6572 bytes
    Balance updates:
      tz1fp5ncDmqYwYC568fREYz9iwQTgGQuKZqX ... -ꜩ0.000956
      payload fees(the block proposer) ....... +ꜩ0.000956
    Smart rollup origination:
      Kind: wasm_2_0_0
      Parameter type: bytes
      Kernel Blake2B hash: '24df9e3c520dd9a9c49b447766e8a604d31138c1aacb4a67532499c6a8b348cc'
      This smart rollup origination was successfully applied
      Consumed gas: 2748.269
      Storage size: 6552 bytes
      Address: sr1RYurGZtN8KNSpkMcCt9CgWeUaNkzsAfXf
      Genesis commitment hash: src13wCGc2nMVfN7rD1rgeG3g1q7oXYX2m5MJY5ZRooVhLt7JwKXwX
      Balance updates:
        tz1fp5ncDmqYwYC568fREYz9iwQTgGQuKZqX ... -ꜩ1.638
        storage fees ........................... +ꜩ1.638

The address sr1RYurGZtN8KNSpkMcCt9CgWeUaNkzsAfXf is the smart rollup address. Let’s write it ${SR_ADDR} from now on.

Deploying a rollup node#

Now that the rollup is originated, anyone can make it progress by deploying a rollup node.

First, we need to decide on a mode the rollup node will run:

  1. operator activates a full-fledged rollup node. This means that the rollup node will do everything needed to make the rollup progress. This includes following the Layer 1 chain, reconstructing inboxes, updating the states, publishing and cementing commitments regularly, and playing the refutation games. In this mode, the rollup node will accept transactions in its queue and batch them on the Layer 1.

  2. batcher means that the rollup node will accept transactions in its queue and batch them on the Layer 1. In this mode, the rollup node follows the Layer 1 chain, but it does not update its state and does not reconstruct inboxes. Consequently, it does not publish commitments nor play refutation games.

  3. observer means that the rollup node follows the Layer 1 chain to reconstruct inboxes, to update its state. However, it will neither publish commitments, nor play a refutation game. It does not include the message batching service either.

  4. maintenance is the same as the operator mode except that it does not include the message batching service.

  5. accuser follows the layer1-chain and computes commitments but does not publish them. Only when a conflicting commitment (published by another committer) is detected will the “accuser node” publish a commitment and participate in the subsequent refutation game.

  6. bailout mode is designed to assist committers in recovering their bonds. It functions as a slightly modified version of “Accuser”, differing in that it does not post any new commitments but instead focuses on defending the ones that have been previously submitted. When operating in bailout mode, the expectation is to initiate a recover bond operation when the operator is no longer staking on any commitment. If the node detects that this operation has been successful, it can gratefully exit.

  7. custom mode refers to a mode where the users individually select which kinds of operations the rollup node injects. It provides tailored control and flexibility customized to specific requirements, and is mostly used for tests.

To each mode corresponds a set of purposes where each purpose is a set of L1 operations which are injected by the rollup node.

The following table links each purpose to its corresponding L1 operations.


smart_rollup_publish, smart_rollup_refute, smart_rollup_timeout









The table below summarises the modes and their associated purposes:











Yes [1]






Yes [1]


Yes [2]






Yes [3]

















Then to run the rollup node, use the following command:

octez-smart-rollup-node --base-dir "${OCLIENT_DIR}" \
                 run "${ROLLUP_NODE_MODE}" \
                 for "${SR_ALIAS_OR_ADDR}" \
                 with operators "${OPERATOR_ADDR}" \
                 --data-dir "${ROLLUP_NODE_DIR}"

where ${OCLIENT_DIR} is the data directory of the Octez client, by default ~/.tezos-client, and ${ROLLUP_NODE_DIR} is the data directory of the Octez smart rollup node, by default ~/.tezos-smart-rollup-node.

The log should show that the rollup node follows the Layer 1 chain and processes the inbox of each level.

Distinct Layer 1 signers can be used for each purpose of the mode by either editing the configuration file or by listing multiple operators on the command line.

For example for the operator mode we can replace ${OPERATOR_ADDR} by default:${OPERATOR_ADDR1} batching:${OPERATOR_ADDR2}. Where the rollup node will use ${OPERATOR_ADDR2} for the batching purpose and ${OPERATOR_ADDR1} for everything else.

The L1 chain has a limitation of one manager operation per key per block (see Prechecking of manager operations). In the case of a high throughput rollup, this limitation could slow down the rollup by capping the number of L2 messages that the rollup node’s batcher purpose can inject per block to the maximum size of one L1 operation’s maximal size (e.g., 32kb on mainnet).

To bypass that limitation and inject multiple smart_rollup_add_messages L1 operations within a single L1 block, it is possible to provide multiple keys for the batcher purpose of a rollup node. At each block, the rollup node will use as many keys as possible to inject a corresponding number of queued L2 messages into the L1 rollup inbox. The order of the batches of L2 messages is not guaranteed to be preserved by the rollup node nor by the octez node mempool.

The way to provide multiple batcher keys on the command line is:

octez-smart-rollup-node run ${ROLLUP_NODE_MODE} for "${SR_ALIAS_OR_ADDR}" \
                 with operators default:${DEFAULT_ADDR} \
                 batching:${BATCHER_ADDR1} \
                 batching:${BATCHER_ADDR2} ...

Configuration file#

The rollup node can also be configured via one configuration file stored in its own data directory, with the following command that uses the same arguments as the run command:

octez-smart-rollup-node --base-dir "${OCLIENT_DIR}" \
                 init "${ROLLUP_NODE_MODE}" config \
                 for "${SR_ALIAS_OR_ADDR}" \
                 with operators "${OPERATOR_ADDR}" \
                 --data-dir "${ROLLUP_NODE_DIR}"

where ${OCLIENT_DIR} must be the directory of the client, containing all the keys used by the rollup node, i.e. ${OPERATOR_ADDR}.

This creates a smart rollup node configuration file:

Smart rollup node configuration written in ${ROLLUP_NODE_DIR}/config.json

Here is the content of the file:

  "smart-rollup-address": "${SR_ALIAS_OR_ADDR}",
    "operating": "${OPERATOR_ADDR}",
    "batching": [ "${OPERATOR_ADDR}" ],
    "cementing": "${OPERATOR_ADDR}",
    "executing_outbox": "${OPERATOR_ADDR}"
  "fee-parameters": {},
  "mode": "operator"

The rollup node can now be run with just:

octez-smart-rollup-node --base-dir "${OCLIENT_DIR}" run --data-dir ${ROLLUP_NODE_DIR}

The configuration will be read from ${ROLLUP_NODE_DIR}/config.json.

Rollup node in a sandbox#

The node can also be tested locally with a sandbox environment. (See sandbox documentation.)

Once you initialized the “sandboxed” client data with ./src/bin_client/, you can run a sandboxed rollup node with octez-smart-rollup-node run.

A temporary directory /tmp/tezos-smart-rollup-node.xxxxxxxx will be used. However, a specific data directory can be set with the environment variable SCORU_DATA_DIR.

History modes#

The rollup node can be configured (1) to remove data on disk that is not needed anymore for the correct operation of a rollup node (i.e. to still be able to play all refutation games that could occur) or (2) to keep the full history of the rollup and the L2 chain since the rollup genesis.

The history mode can be set on the command line with --history-mode <mode> or in the configuration file with:

  "history-mode" : "<mode>"

Full mode#

The full history mode makes the rollup node keep its history since the last cemented commitment (LCC). Everything before the LCC (both the context containing the PVM state and the rollup node store containing the L2 chain) is automatically deleted periodically by a garbage collection phase.

Archive mode#

When configured in archive mode, a rollup node will keep all history since the origination of the rollup. This mode can be useful for applications that require to regularly access historical data before the LCC, i.e. for applications that need more than two weeks of history.

This mode can be chosen e.g. on the command line with --history-mode archive.

Note that an archive node can be converted to a full node but not the other way around. The conversion will happen automatically if the history mode is changed in the configuration file or command line.

This is the default history mode.


Smart rollup node snapshots are a way to bootstrap a rollup node without having to replay the whole L2 chain since the rollup genesis. Without this snapshot mechanism, one would need an archive L1 node to bootstrap a rollup node or to catch up (if the rollup node data is more than a few days old) because it needs access to metadata about L1 operations on the chain.

Snapshots for a particular rollup must be obtained from an off-chain source (for instance a rollup snapshot provider service which regularly publishes snapshots online) and imported into an existing, or empty, rollup node to get started quickly.

Format of snapshots#

A smart rollup node snapshot is a binary file which contains a header part and a data part. The data part is a tar archive of the non-local storage files of the rollup node while the metadata header exposes information to quickly validate or discard a snapshot.

Snapshot format#




Snapshot version

1 byte

0 (the only version so far)

History mode

1 byte

0 for archive, 1 for full


20 bytes

Address of the smart rollup

Head level

4 bytes

Level of the last seen L1 block of the rollup (int32)

Last commitment hash

32 bytes

Hash of last commitment in the L2 chain



Tar archive of rollup node data

The snapshots can be imported (and exported) as either compressed (with gzip) or uncompressed files.

Importing a snapshot#

A snapshot ${SNAPSHOT_FILE} can be imported by issuing the following command:

octez-smart-rollup-node -E ${L1_NODE_ENDPOINT} \
  snapshot import ${SNAPSHOT_FILE} \
  [--data-dir ${ROLLUP_NODE_DIR}] \

where ${ROLLUP_NODE_DIR} is the data directory of the rollup node in which we want to import the snapshot, and ${L1_NODE_ENDPOINT} is the RPC endpoint of an L1 node, needed to verify the snapshot.

Option --force allows to import a snapshot in an already populated data directory of a rollup node.


When using the --force option, it is recommended to run the snapshot info command and to first import the snapshot in an empty directory to run all the checks.

While importing a snapshot, many checks are performed to ensure the consistency of the imported data. In order to speed up the process, but only if the snapshot’s source is highly trusted (or exported by yourself), it is possible to disable some checks. Some rudimentary checks will still be performed. However, most of the data will be copied directly, without additional consistency checks. To do so, use the --no-check option.


The snapshot importing mechanism checks that the chain of commitments from the LCC (last cemented commitment) to the last commitment is published on the L1 chain but this does not prevent a malicious provider of snapshots from providing snapshots with inaccurate data about the L2 state, as soon as she or he is willing to also forfeit her/his deposit (these commitments were exposed on L1 to eventual refutation games). The check described above gives some acceptable level of assurance without having to recompute the whole chain from the LCC (which can be costly depending on the rollup).

List of checks performed for import#

  • Metadata checks:

    • Rollup address matches (*)

    • History mode matches (*)

    • Snapshot head is fresher than the one on disk (*)

    • Last commitment is published on L1

  • Metadata commitment matches the snapshot one

  • LCC on L1 is a valid commitment in the snapshot

  • Ensure the chain of commitments goes back to LCC

  • For each L2 block:

    • The commitment, if any, must be for the PVM state of this block

    • Hashes are for the correct content (for state hash, commitment hash, inbox hash)

(*) Marks the rudimentary checks that are performed on import with option --no-checks.

Snapshot information#

When retrieving a snapshot, it can be useful to check its actual content, such as:

  • snapshot format

  • history mode

  • smart rollup address

  • head level

  • last commitment

This information is displayed by the following command:

octez-smart-rollup-node snapshot info ${SNAPSHOT_FILE}

which will essentially decode and display the metadata header of the snapshot file.

Exporting a snapshot#

Exporting a snapshot for a currently running rollup node will temporarily stop its progression (during the time the data is initially copied). The export creates a file with a chosen name ${SNAPSHOT_FILE} or one which is automatically generated of the form snapshot-<address>-<level>.<history_mode> and is achieved by running the following command (the rollup node does not need to be stopped):

octez-smart-rollup-node snapshot export [${SNAPSHOT_FILE}] \
  [--data-dir ${ROLLUP_NODE_DIR}] \
  [--dest ${DEST_DIR}]

The export has three phases:

  1. Initial export of the data (blocking)

  2. Compression of snapshot (non-blocking)

  3. Integrity check of snapshot (non-blocking)

The checks for the export are less thorough than the ones for an import but ensure that the snapshot is not corrupted and can be imported by other users.


It is also possible to use the --no-check option to disable the integrity checks during the export (i.e., phase 3), which will speed up the process.


Snapshots produced with the --compact option will be significantly smaller (by a factor of 3) than otherwise as they contain a single commit of the context for the first available block (instead of the full context history). They take a comparable amount of time to be exported but take longer to be imported because the context needs to be reconstructed.


Snapshots exported with --compact for archive rollup nodes will need a significant time to import because the context will need to be reconstructed from the rollup genesis.


Sending an external inbox message#

The Octez client can be used to send an external message into the rollup inbox. Assuming that ${EMESSAGE} is the hexadecimal representation of the message payload, one can do:

octez-client -d "${OCLIENT_DIR}" -p ${PROTO_HASH} \
 send smart rollup message "hex:[ \"${EMESSAGE}\" ]" \
 from "${OPERATOR_ADDR}"

to inject such an external message, where ${PROTO_HASH} is the hash of your protocol (e.g. ProtoALphaAL for Alpha; see how to obtain it). So let us focus now on producing viable content for ${EMESSAGE}.

The kernel used previously in our running example is a simple “echo” kernel that copies its input as a new message to its outbox. Therefore, the input must be a valid binary encoding of an outbox message to make this work. Specifically, assuming that we have originated a Layer 1 smart contract as follows:

octez-client -d "${OCLIENT_DIR}" -p ${PROTO_HASH} \
  originate contract go transferring 1 from "${OPERATOR_ADDR}" \
  running 'parameter string; storage string; code {CAR; NIL operation; PAIR};' \
  --init '""' --burn-cap 0.4

and that this contract is identified by an address ${CONTRACT} (a KT1... address), then one can encode an outbox transaction using the octez-codec as follows:

    "transactions": [
        "parameters": {"int": "37"},
        "destination": "KT1VD4SdQF2ruNNTCE1aTWErmGz9tN4Mg8F5",
        "entrypoint": "%default"
    "kind": "untyped"

EMESSAGE=$(octez-codec encode alpha.smart_rollup.outbox.message from "${MESSAGE}")

Triggering the execution of an outbox message#

Once an outbox message has been pushed to the outbox by the kernel at some level ${L}, the user needs to wait for the commitment that includes this level to be cemented. On Weeklynet, the cementation process of a non-disputed commitment is 40 blocks long while on Mainnet, it is 2 weeks long.

When the commitment is cemented, one can observe that the outbox is populated as follows:

curl -i "${ROLLUP_NODE_ENDPOINT}/global/block/cemented/outbox/${L}/messages"


  • ${ROLLUP_NODE_ENDPOINT} represents the address of the Rollup node server. It can be set to a specific server address such as “http://localhost:36149”.

  • ${L} denotes the block level for which one wants to retrieve information from the Rollup node.

Here is the output for this command:

[ { "outbox_level": ${L}, "message_index": "0",
   { "transactions":
       [ { "parameters": { "int": "37" },
           "destination": "${CONTRACT}",
           "entrypoint": "%default" } ] } } ]

At this point, the actual execution of a given outbox message can be triggered. This requires precomputing a proof that this outbox message is indeed in the outbox. In the case of our running example, this proof is retrieved as follows:

PROOF=$(curl -i "${ROLLUP_NODE_ENDPOINT}/global/block/head/helpers/\

Finally, the execution of the outbox message is done as follows:

"${TEZOS_PATH}/octez-client" -d "${OCLIENT_DIR}" -p ${PROTO_HASH} \
        execute outbox message of smart rollup "${SR_ALIAS_OR_ADDR}" \
        from "${OPERATOR_ADDR}" for commitment hash "${LCC}" \
        and output proof "${PROOF}"

where ${LCC} is the hash of the latest cemented commitment. Notice that anyone can trigger the execution of an outbox message (not only an operator as in this example).

One can check in the receipt that the contract has indeed been called with the parameter "37" through an internal operation. More complex parameters, typically containing assets represented as tickets, can be used as long as they match the type of the entrypoint of the destination smart contract.

Sending an internal inbox message#

A smart contract can push an internal message in the rollup inbox using the Michelson TRANSFER_TOKENS instruction targeting a specific rollup address. The parameter of this transfer must be a value of the Michelson type declared at the origination of this rollup.

Remember that our running example rollup has been originated with:

octez-client originate smart rollup "${SR_ALIAS}" \
  from "${OPERATOR_ADDR}" \
  of kind wasm_2_0_0 \
  of type bytes \
  booting with "${KERNEL}" \
  -burn-cap 999

The fragment of type bytes of this command declares that the rollup is expecting values of type bytes. (Notice any Michelson type could have been used instead. To transfer tickets to a rollup, this type must mention tickets.)

Here is an example of a Michelson script that sends an internal message to the rollup of our running example. The payload of the internal message is the value passed as parameter of type bytes to the rollup.

parameter bytes;
storage unit;
    PUSH address "${SR_ADDR}";
    CONTRACT bytes;
    IF_NONE { PUSH string "Invalid address"; FAILWITH } {};
    PUSH mutez 0;
    DIG 2;
    NIL operation;

Populating the reveal channel#

It is the responsibility of rollup node operators to get the data passed through the reveal data channel when the rollup requests it.

To answer a request for a page of hash H, the rollup node tries to read the content of a file H named ${ROLLUP_NODE_DIR}/wasm_2_0_0.

Notice that a page cannot exceed 4KB. Hence, larger pieces of data must be represented with multiple pages that reference each other through hashes. It is up to the kernel to decide how to implement this. For instance, one can classify pages into two categories: index pages that are hashes for other pages and leaf pages that contain actual payloads.

In addition to data stored locally on disk in the data directory, the rollup node can also fetch missing pages to be revealed from an external source. Data fetched from a remote pre-images service will be cached on disk (in the ${ROLLUP_NODE_DIR}/wasm_2_0_0).

To configure a remote source (whose server can be contacted over HTTP at ${PRE_IMAGES_URL}) for pre-images, one can either pass the option --pre-images-endpoint ${PRE_IMAGES_URL} to the run command or add the following in the configuration file ${ROLLUP_NODE_DIR}/config.json:

  "pre-images-endpoint" : "${PRE_IMAGES_URL}"


One does not need to trust the provider service for pre-images because the rollup node will ensure that the content matches the requested hash before using it.

Configure WebAssembly fast execution#

When the rollup node advances its internal rollup state under normal operation, it does so using the fast execution engine.

This engine uses Wasmer for running WebAssembly code. You may configure the compiler used for compiling WebAssembly code, via the OCTEZ_WASMER_COMPILER environment variable.

The choice of a compiler primarily affects the performance of the WebAssembly code execution vs the compilation time. Some compilers offer certain security guarantees in a blockchain context, such as compiling in linear time to avoid JIT bombs.

The available options are:

Wasmer compiler options#






When to use Singlepass



When to use Cranelift

Note that while the rollup node is generally capable of using Wasmer’s LLVM-based compiler, Octez does not currently ship with it.

When the environment variable is undefined, Cranelift is used by default.

Developing WASM Kernels#

This page provides a first overview on writing a Wasm kernel for a smart rollup. (See smart optimistic rollup)

A rollup is primarily characterized by the semantics it gives to the input messages it processes. This semantics is provided at origination time as a WASM program (in the case of the wasm_2_0_0 kind) called a kernel. More concretely, the kernel is a WASM module encoded in the binary format defined by the WASM standard.

Except for necessary restrictions to ensure determinism (a key requirement for any web3 technology), we support the full WASM language. More precisely, determinism is ensured by the following restrictions:

  1. Instructions and types related to floating-point arithmetic are not supported. This is because IEEE floats are not deterministic, as the standard includes undefined behavior operations.

  2. The length of the call stack of the WASM kernel is bounded.

Modulo the limitations above, a valid kernel is a WASM module that satisfies the following constraints:

  1. It exports a function kernel_run that takes no argument and returns nothing.

  2. It declares and exports exactly one memory.

  3. It only imports the host functions exported by the (virtual) module smart_rollup_core.

For instance, the mandatory example of a hello, world! kernel is the following WASM program in text format.

  (import "smart_rollup_core" "write_debug"
     (func $write_debug (param i32 i32) (result i32)))
  (memory 1)
  (export "mem" (memory 0))
  (data (i32.const 100) "hello, world!")
  (func (export "kernel_run")
    (local $hello_address i32)
    (local $hello_length i32)
    (local.set $hello_address (i32.const 100))
    (local.set $hello_length (i32.const 13))
    (drop (call $write_debug (local.get $hello_address)
                             (local.get $hello_length)))))

This program can be compiled to the WASM binary format with general-purpose tool like WABT.

wat2wasm hello.wat -o hello.wasm

The contents of the resulting hello.wasm file is a valid WASM kernel, though its relevance as a decentralized application is debatable.

One of the benefits of choosing WASM as the programming language for smart rollups is that WASM has gradually become a ubiquitous compilation target over the years. Its popularity has grown to the point where mainstream, industrial languages like Go or Rust now natively compile to WASM. Thus, cargo —the official Rust package manager— provides an official target to compile Rust to .wasm binary files, which are valid WASM kernels. This means that, for this particular example, one can build a WASM kernel while enjoying the strengths and convenience of the Rust language and the Rust ecosystem.

The rest of the section proceeds as follows.

  1. First, we explain the execution environment of a WASM kernel: when it is parsed, executed, etc.

  2. Then, we explain in more detail the API at the disposal of WASM kernel developers.

  3. Finally, we demonstrate how Rust in particular can be used to implement a WASM kernel.

Though Rust has become the primary language whose WASM backend has been tested in the context of smart rollups, the WASM VM has not been modified in any way to favor this language. We fully expect that other mainstream languages such as Go are also good candidates for implementing WASM kernels.

Execution Environment#

In a nutshell, the life cycle of a smart rollup is a never-ending loop of fetching inputs from the Layer 1, and executing the kernel_run function exposed by the WASM kernel.


The smart rollup carries two states:

  1. A transient state, that is reset after each call to the kernel_run function and is akin to RAM.

  2. A persistent state, that is preserved across kernel_run calls. The persistent state consists in an inbox that is regularly populated with the inputs coming from the Layer 1, the outbox which the kernel can populate with contract calls targeting smart contracts in the Layer 1, and a durable storage which is akin to a file system.

The durable storage is a persistent tree, whose contents are addressed by path-like keys. A path in the storage may contain: a value (also called file) consisting of a sequence of raw bytes, and/or any number of subtrees (also called directories), that is, the paths in the storage prefixed by the current path. Thus, unlike most file systems, a path in the durable storage may be at the same time a file and a directory (a set of sub-paths).

The WASM kernel can write and read the raw bytes stored under a given path (the file), but can also interact (delete, copy, move, etc.) with subtrees (directories).

The values and subtrees under the key /readonly are not writable by a kernel, but can be used by the PVM to give information to the kernel.

WASM PVM Versioning#

One of Tezos distinguishing features is its native support for upgrades. At its core, Tezos is a Layer 1 designed to evolve via a self-updating mechanism, subject to an on-line governance process. The self-updating mechanism is also implemented by the smart rollup infrastructure.

The WASM PVM is versioned. Kernels can read the version of the underlying WASM PVM (which is currently interpreting them) by reading the contents of the file stored under the key /readonly/wasm_version in their durable storage.

New WASM PVM versions are introduced by new Layer 1’s protocol upgrades. The WASM PVM will upgrade itself when it reads the Protocol_migration internal message.













The changes in each WASM PVM version can be found by searching for string “PVM” in the corresponding protocol’s changelog, section Smart Rollups (e.g. this section for protocol Alpha).

Control Flow#

When a new block is published on Tezos, the inbox exposed to the smart rollup is populated with all the inputs published on Tezos in this block. It is important to keep in mind that all the smart rollups which are originated on Tezos share the same inbox. As a consequence, a WASM kernel has to filter the inputs that are relevant to its purpose from the ones it does not need to process.

Once the inbox has been populated with the inputs of the Tezos block, the kernel_run function is called, from a clean “transient” state. More precisely, the WASM kernel is re-initialized, then kernel_run is called.

By default, the WASM kernel yields when kernel_run returns. In this case, the WASM kernel execution is put on hold while the inputs of the next inbox are being loaded. The inputs that were not consumed by kernel_run are dropped. kernel_run can prevent the WASM kernel from yielding by writing arbitrary data under the path /kernel/env/reboot in its durable storage. In such a case (known as reboot), kernel_run is called again, without dropping unread inputs. The value at /kernel/env/reboot is removed between each call of kernel_run, and the kernel_run function can postpone yielding at most 1,000 reboots for each Tezos level.

A call to kernel_run cannot take an arbitrary amount of time to complete, because diverging computations are not compatible with the optimistic rollup infrastructure of Tezos. It is the responsibility of the kernel developers to ensure the computation time necessary to track their rollup is bounded and reasonable, for two reasons:

  1. If a kernel requires more time than the time between two Tezos blocks, then a rollup node is doomed to lag behind the Layer 1 chain it is tracking.

  2. If a single kernel_run takes too much time to compute, then it becomes difficult to protect the resulting commitment. This is because the WASM PVM interpreter has not been optimized for speed but for producing small execution step proofs.

Kernel developers are expected to design their kernel such that it addresses these two constraints.

The WASM PVM does enforce a limit on the number of ticks available per kernel_run, but it is arbitrary high enough (50 trillion) that it becomes virtually impossible to exceed it. octez-smart-rollup-wasm-debugger is probably the best tool available to verify the kernel_run function does not take more ticks than authorized.

The direct consequence of this setup is that it might be necessary for a WASM kernel to span a long computation across several calls to kernel_run, and therefore to serialize any data it needs in the durable storage to avoid losing them.

Finally, the kernel can verify if the previous kernel_run invocation was trapped by verifying if some data are stored under the path /kernel/env/stuck.

Host Functions#

At its core, the WASM machine defined in the WASM standard is just a very evolved arithmetic machine. It needs to be enriched with so-called host functions in order to be used for greater purposes. The host functions provide an API to the WASM program to interact with an “outer world”.

As for smart rollups, the host functions exposed to a WASM kernel allow it to interact with the components of persistent state:


Loads the oldest input still present in the inbox of the smart rollup in the transient memory of the WASM kernel. This means that the input is lost at the next invocation of kernel_run if it is not written in the durable storage. Since version 2.0.0 of the WASM PVM.


Writes an in-memory buffer to the outbox of the smart rollup. If the content of the buffer follows the expected encoding, it can be interpreted in the Layer 1 as a smart contract call, once a commitment acknowledging the call to this host function is cemented. Since version 2.0.0 of the WASM PVM.


Can be used by the WASM kernel to log events which can potentially be interpreted by an instrumented rollup node. Since version 2.0.0 of the WASM PVM.


Returns the kind of data (if any) stored in the durable storage under a given path: a directory, a file, neither or both. Since version 2.0.0 of the WASM PVM.


Cuts both the value (if any) and any subdirectory under a given path out of the durable storage. Since version 2.0.0 of the WASM PVM.


Cuts the value under a given path out of the durable storage, but leaves the rest of the subtree untouched. Since version 2.0.0-r1 of the WASM PVM.


Copies the subtree under a given path to another key. Since the 2.0.0 version of the WASM PVM.


Behaves as store_copy, but also cuts the original subtree out of the tree. Since version 2.0.0 of the WASM PVM.


Loads at most 4,096 bytes from a file of the durable storage to a buffer in the memory of the WASM kernel. Since version 2.0.0 of the WASM PVM.*


Writes at most 2048 bytes from a buffer in the memory of the WASM kernel to a file of the durable storage, increasing its size if necessary. Note that files in the durable storage cannot exceed \(2^{31} - 1\) bytes (i.e. 2GB - 1). Since the 2.0.0 version of the WASM PVM.


Allocates a new file in the durable storage under a given key. Similarly to store_write, store_create cannot create files larger than the durable storage limits, that is 2GB - 1. Since the 2.0.0-r1 of the WASM PVM.


Returns the size (in bytes) of a file under a given key in the durable storage. Since version 2.0.0 of the WASM PVM.


Returns the number of child objects (either directories or files) under a given key. Since version 2.0.0 of the WASM PVM.


Loads in memory the preimage of a hash. The size of the hash in bytes must be specified as an input to the function. Since the 2.0.0 version of the WASM PVM.


Loads in memory the address of the smart rollup (20 bytes), and the Tezos level of its origination (4 bytes). Since the 2.0.0 version of the WASM PVM.

These host functions use a “C-like” API. In particular, most of them return a signed 32bit integer, where negative values are reserved for conveying errors, as shown in the next table.




Input is too large to be a valid key of the durable storage


Input cannot be parsed as a valid key of the durable storage


There is no file under the requested key


The host functions tried to read or write an invalid section (determined by an offset and a length) of the value stored under a given key


Cannot write a value beyond the 2GB size limit


Invalid memory access (segmentation fault)


Tried to read from the inbox or write to the outbox more than 4,096 bytes


Unknown error due to an invalid access


Attempt to modify a readonly value


Key has no tree in the storage


Outbox is full, no new message can be appended


Key has already a value in the storage

Implementing a WASM Kernel in Rust#

Though WASM is a good fit for efficiently executing computation-intensive, arbitrary programs, it is a low-level, stack-based, memory unsafe language. Fortunately, it was designed to be a compilation target, not a language in which developers would directly write their programs.

Rust has several advantages that make it a good candidate for writing the kernel of a smart rollup. Not only does the Rust compiler treat WASM as a first class citizen when it comes to compilation targets, but its approach to memory safety eliminates large classes of bugs and vulnerabilities that arbitrary WASM programs may suffer from.

Additionally there is support for implementing kernels in Rust, in the form of the Smart Rollup Kernel SDK.

Setting-up Rust#

rustup is the standard way to get Rust. Once rustup is installed, enabling WASM as a compilation target is as simple as running the following command.

rustup target add wasm32-unknown-unknown

Rust also proposes the wasm64-unknown-unknown compilation target. This target is not compatible with Tezos smart rollups, which only provide a 32bit address space.


This document is not a tutorial about Rust, and familiarity with the language and its ecosystem (e.g., how Rust crates are structured in particular) is assumed.

The simplest kernel one can implement in Rust (the one that returns directly after being called, without doing anything particular) is the following Rust file (by convention named in Rust).

pub extern "C" fn kernel_run() {

This code can be easily computed with cargo with the following Cargo.toml.

name = 'noop'
version = '0.1.0'
edition = '2021'

crate-type = ["cdylib"]

The key line to spot is the crate-type definition to cdylib. As a side note, when writing a library that will eventually be consumed by a Kernel WASM crate, this line must be modified to

crate-type = ["cdylib", "rlib"]

Compiling our “noop” kernel is done by calling cargo with the correct argument.

cargo build --target wasm32-unknown-unknown

It is also possible to use the --release CLI flag to tell cargo to optimize the kernel.

To make the use of the target optional, it is possible to create a .cargo/config.toml file, containing the following line.

target = "wasm32-unknown-unknown"

lld = true

The resulting project looks as follows.

├── .cargo
│   └── config.toml
├── Cargo.toml
└── src

and the kernel can be found in the target/ directory, e.g., ./target/wasm32-unknown-unknown/release/noop.wasm.

By default, Rust binaries (including WASM binaries) contain a lot of debugging information and possibly unused code that we do not want to deploy in our rollup. For instance, our “noop” kernel weighs 1.7MBytes. We can use wasm-strip to reduce the size of the kernel (down to 115 bytes in our case).

Host Functions in Rust#

The host functions exported by the WASM runtime to Rust programs are exposed by the following API. The link pragma is used to specify the module that exports them (in our case, smart_rollup_core). Define these functions in the as follows:

pub struct ReadInputMessageInfo {
    pub level: i32,
    pub id: i32,

#[link(wasm_import_module = "smart_rollup_core")]
extern "C" {
    /// Returns the number of bytes written to `dst`, or an error code.
    pub fn read_input(
        message_info: *mut ReadInputMessageInfo,
        dst: *mut u8,
        max_bytes: usize,
    ) -> i32;

    /// Returns 0 in case of success, or an error code.
    pub fn write_output(src: *const u8, num_bytes: usize) -> i32;

    /// Does nothing. Does not check the correctness of its argument.
    pub fn write_debug(src: *const u8, num_bytes: usize);

    /// Returns
    /// - 0 the key is missing
    /// - 1 only a file is stored under the path
    /// - 2 only directories under the path
    /// - 3 both a file and directories
    pub fn store_has(path: *const u8, path_len: usize) -> i32;

    /// Returns 0 in case of success, or an error code
    pub fn store_delete(path: *const u8, path_len: usize) -> i32;

    /// Returns the number of children (file and directories) under a
    /// given key.
    pub fn store_list_size(path: *const u8, path_len: usize) -> i64;

    /// Returns 0 in case of success, or an error code.
    pub fn store_copy(
        src_path: *const u8,
        scr_path_len: usize,
        dst_path: *const u8,
        dst_path_len: usize,
    ) -> i32;

    /// Returns 0 in case of success, or an error code.
    pub fn store_move(
        src_path: *const u8,
        scr_path_len: usize,
        dst_path: *const u8,
        dst_path_len: usize,
    ) -> i32;

    /// Returns the number of bytes written to the durable storage
    /// (should be equal to `num_bytes`, or an error code).
    pub fn store_read(
        path: *const u8,
        path_len: usize,
        offset: usize,
        dst: *mut u8,
        num_bytes: usize,
    ) -> i32;

    /// Returns 0 in case of success, or an error code.
    pub fn store_write(
        path: *const u8,
        path_len: usize,
        offset: usize,
        src: *const u8,
        num_bytes: usize,
    ) -> i32;

    /// Returns the number of bytes written at `dst`, or an error
    /// code.
    pub fn reveal_metadata(
        dst: *mut u8,
        max_bytes: usize,
    ) -> i32;

    /// Returns the number of bytes written at `dst`, or an error
    /// code.
    pub fn reveal_preimage(
        hash_addr: *const u8,
        hash_size: u8,
        dst: *mut u8,
        max_bytes: usize,
    ) -> i32;

These functions are marked as unsafe for Rust. It is possible to provide a safe API on top of them. For instance, the read_input host function can be used to declare a safe function which allocates a fresh Rust Vector to receive the input.

Define these functions in the as follows:

// Assuming the host functions are defined in a module `host`.

mod host;
use crate::host::read_input;
use crate::host:ReadInputMessageInfo;

pub const MAX_MESSAGE_SIZE: u32 = 4096u32;

pub struct Input {
    pub level: u32,
    pub id: u32,
    pub payload: Vec<u8>,

pub fn next_input() -> Option<Input> {
    let mut payload = Vec::with_capacity(MAX_MESSAGE_SIZE as usize);

    // Placeholder values
    let mut message_info = ReadInputMessageInfo { level: 0, id: 0 };

    let size = unsafe {
            &mut message_info,

    if 0 < payload.len() {
        unsafe { payload.set_len(size as usize) };
        Some(Input {
            level: message_info.level as u32,
            id: as u32,
    } else {

Coupling Vec::with_capacity along with the set_len unsafe function is a good approach to avoid initializing the 4,096 bytes of memory every time you want to load data of arbitrary size into the WASM memory.

Testing your Kernel#


octez-smart-rollup-wasm-debugger is available in the Octez distribution starting with Version 16.1.

Testing a kernel without having to start a rollup node on a test network is very convenient. We provide a debugger as a means to evaluate the WASM PVM without relying on any node and network: octez-smart-rollup-wasm-debugger.

octez-smart-rollup-wasm-debugger --kernel "${WASM_FILE}" --inputs "${JSON_INPUTS}" --rollup "${SR_ADDR}"

octez-smart-rollup-wasm-debugger takes the target WASM kernel to be debugged as argument, either as a .wasm file (the binary representation of WebAssembly modules) or as a .wast file (its textual representation), and actually parses and typechecks the kernel before giving it to the PVM.

Beside the kernel file, the debugger can optionally take an input file containing inboxes and a rollup address. The expected contents of the inboxes is a JSON value, with the following schema:

  [ { "payload" : <Michelson data>,
      "sender" : <Smart contract sending to the rollup, optional>,
      "source" : <User account sending the message, optional>
      "destination" : <Smart rollup address, optional> }
    // or
    { "external" : <hexadecimal payload> }

The contents of the input file is a JSON array of arrays of inputs, which encodes a sequence of inboxes, where an inbox is a set of messages. These inboxes are read in the same order as they appear in the JSON file. For example, here is a valid input file that defines two inboxes: the first array encodes an inbox containing only an external message, while the second array encodes an inbox containing two messages:

      "payload" : "0",
      "sender" : "KT1ThEdxfUcWUwqsdergy3QnbCWGHSUHeHJq",
      "source" : "tz1RjtZUVeLhADFHDL8UwDZA6vjWWhojpu5w",
      "destination" : "sr1RYurGZtN8KNSpkMcCt9CgWeUaNkzsAfXf"
    { "payload" : "Pair Unit False" }

Note that the sender, source and destination fields are optional and will be given default values by the debugger, respectively KT18amZmM5W7qDWVt2pH6uj7sCEd3kbzLrHT, tz1Ke2h7sDdakHJQh8WX4Z372du1KChsksyU and sr163Lv22CdE8QagCwf48PWDTquk6isQwv57. If no input file is given, the inbox will be assumed empty. If the option --rollup is given, it replaces the default value for the rollup address.

octez-smart-rollup-wasm-debugger is a debugger, as such it waits for user inputs to continue its execution. Its initial state is exactly the same as right after its origination. Its current state can be inspected with the command show status:

> show status
Status: Waiting for input
Internal state: Collect

When started, the kernel is in collection mode internally. This means that it is not executing any WASM code, and is waiting for inputs in order to proceed. The command load inputs will load the first inbox from the file given with the option --inputs, putting Start_of_level and Info_per_level before these inputs and End_of_level after the inputs.

> load inputs
Loaded 1 inputs at level 0

> show status
Status: Evaluating
Internal state: Snapshot

At this point, the internal input buffer can be inspected with the command show inbox.

> show inbox
Inbox has 4 messages:
{ raw_level: 0;
  counter: 0
  payload: Start_of_level }
{ raw_level: 0;
  counter: 1
  payload: Info_per_level {predecessor_timestamp = 1970-01-01T00:00:00-00:00; predecessor = BKiHLREqU3JkXfzEDYAkmmfX48gBDtYhMrpA98s7Aq4SzbUAB6M} }
{ raw_level: 0;
  counter: 2
  payload: 0000000023030b01d1a37c088a1221b636bb5fccb35e05181038ba7c000000000764656661756c74 }
{ raw_level: 0;
  counter: 3
  payload: End_of_level }

The first input of an inbox at the beginning of a level is Start_of_level, and is represented by the message \000\001 on the kernel side. We can now start a kernel_run evaluation:

> step kernel_run
Evaluation took 11000000000 ticks so far
Status: Waiting for input
Internal state: Collect

The memory of the interpreter is flushed between two kernel_run calls (at the Snapshot and Collect internal states), however the durable storage can be used as a persistent memory. Let’s assume this kernel wrote data at key /store/key:

> show key /store/key
`<hexadecimal value of the key>`

Since the representation of values is decided by the kernel, the debugger can only return its raw value. Please note that the command show keys <path> will return the keys under the given path. This can help navigate in the durable storage.

> show keys /store

It is also possible to inspect the memory by stopping the PVM before its snapshot internal state, with step result, and inspect the memory at pointer n and length l, and finally evaluate until the next kernel_run:

> step result
Evaluation took 2500 ticks so far
Status: Evaluating
Internal state: Evaluation succeeded

> show memory at p for l bytes
`<hexadecimal value>`

> step kernel_run
Evaluation took 7500 ticks so far
Status: Evaluating
Internal state: Snapshot

Once again, note that values from the memory are output as is, since the representation is internal to WASM.

Finally, it is possible to evaluate the whole inbox with step inbox. It will take care of the possible reboots asked by the kernel (through the usage of the /kernel/env/reboot_flag flag) and stop at the next collection phase.

> step inbox
Evaluation took 44000000000 ticks
Status: Waiting for input
Internal state: Collect

To obtain more information on the execution, the command profile will also run the kernel on a full inbox, consume all inputs, run until more inputs are required, and output some information about the run.

> profile
Starting the profiling until new messages are expected. Please note that it will take some time and does not reflect a real computation time.
Profiling result can be found in /tmp/wasm-debugger-profiling-2023-09-26T09:10:09.860-00:00.out
Detailed results for a `kernel_run`:
%interpreter(decode): 35948 ticks (277ms)
%interpreter(link): 6 ticks (3.605us)
%interpreter(init): 201823 ticks (62.246ms)
kernel_run: 22962 ticks (20.280ms)

Full execution: 260739 ticks (359ms)
Detailed results for a `kernel_run`:
%interpreter(decode): 35948 ticks (273ms)
%interpreter(link): 6 ticks (7.287us)
%interpreter(init): 201823 ticks (63.946ms)
kernel_run: 29388 ticks (9.275ms)

Full execution: 267165 ticks (346ms)
Full execution with padding: 22000000000 ticks

Each cycle is a call of the kernel_run function. For each cycle, the number of effective ticks used is shown (ticks corresponding to execution, and not used for padding), along with the duration in seconds.

It is also possible to show the outbox for any given level (show outbox at level 0)

> show outbox at level 0
Outbox has N messages:
{ unparsed_parameters: ..;
  destination: ..;
  entrypoint: ..; }

The reveal channel described previously is available in the debugger, either automatically or through specific commands. The debugger can fill automatically preimages from files in a specific directory on the disk, by default in the preimage subdirectory of the working directory. It can be configured with the option --preimage-dir <directory>. In case there is no corresponding file found for the requested preimage, the debugger will ask for the hexadecimal value of the preimage:

> step inbox
Preimage for hash 0000[..] not found.
> 48656c6c6f207468657265210a
Hello there!

Metadata are automatically filled with level 0 as origination level and the configured smart rollup address (or the default one).

Note that when stepping tick by tick (using the step tick command), it is possible to end up in a situation where the evaluation stops on Waiting for reveal. If the expected value is a metadata, the command reveal metadata will give the default metadata to the kernel. If the value expected is the preimage of a given hash, there are two possible solutions:

  • reveal preimage to read the value from the disk. In that case, the debugger will look for a file of the same name as the expected hash in the preimage subdirectory.

  • reveal preimage of <hex encoded value> can be used to feed a custom preimage hash.