# How to start reading protocol Alpha¶

Protocol Alpha is the name of the initial economic protocol. Alpha is a placeholder name and is not related to the old Alphanet test network.

Before reading that document, you may want to:

As all protocols, Alpha is made of a series of OCaml interface and implementation files, accompanied by a TEZOS_PROTOCOL file.

## The TEZOS_PROTOCOL structure¶

If you look at this file in the repository, you will see that it is composed of the hash of the sources, and the list of its modules, in linking order.

Protocol Alpha is structured as a tower of abstraction layers, a coding discipline that we designed to have OCaml check as many invariants as possible at typing time. You will also see empty lines in TEZOS_PROTOCOL that denote these layers of abstraction.

These layers follow the linking order: the first modules are the tower’s foundation that talk to the raw key-value store, and going forward in the module list means climbing up the abstraction tower.

## The big abstraction barrier: Alpha_context¶

The proof-of-stake algorithm, as described in the white paper, relies on an abstract state of the ledger, that is read and transformed during validation of a block.

Due to the polymorphic nature of Tezos, the ledger’s state (that we call context in the code), cannot be specific to protocol Alpha’s need. The proof-of-stake is thus implemented over a generic key-value store whose keys and associated binary data must implement the abstract structure.

The Alpha_context module enforces the separation of concerns between, on one hand, mapping the abstract state of the ledger to the concrete structure of the key-value store, and, on the other hand, implementing the proof-of-stake algorithm over this state.

In more practical terms, Alpha_context defines a type t that represents a state of the ledger. This state is an abstracted out version of the key-value store that can only be manipulated through the use of the few selected manipulations reexported by Alpha_context, that always preserve the well-typed aspect and internal consistency invariants of the state.

When validating a block, the low-level state that result from the predecessor block is read from the disk, then abstracted out to a Alpha_context.t, which is then only updated by high level operations that preserve consistency, and finally, the low level state is extracted to be committed on disk.

This way, we have two well separated parts in the code. The code below Alpha_context implements the ledger’s state storage, while the code on top of it is the proof-of-stake algorithm. Thanks to this barrier, the latter can remain nice, readable OCaml that only manipulates plain OCaml values.

## Below the Alpha_context¶

For this part, in a first discovery of the source code, you can start by relying mostly on this coarse grained description, with a little bit of cherry-picking when you’re curious about how a specific invariant is enforced.

### The *_repr modules¶

These modules abstract the values of the raw key-value context by using Data_encoding.

These modules define the data types used by the protocol that need to be serialized (amounts, contract handles, script expressions, etc.). For each type, it also defines its serialization format using Data_encoding.

Above this layer, the code should never see the byte sequences in the database, the ones of transmitted blocks and operations, or the raw JSON of data transmitted via RPCs. It only manipulates OCaml values.

### The Storage module and storage functors¶

Even with the concrete formats of values in the context abstracted out, type (or consistency) errors can still occur if the code accesses a value with a wrong key, or a key bound to another value. The next abstraction barrier is a remedy to that.

The storage module is the single place in the protocol where key literals are defined. Hence, it is the only module necessary to audit, to know that the keys are not colliding.

It also abstracts the keys, so that each kind of key get its own accessors. For instance, module Storage.Contract.Balance contains accessors specific to contracts’ balances.

Moreover, the keys bear the type of the values they point to. For instance, only values of type Tez_repr.t can by stored at keys Storage.Contract.Balance. And in case a key is not a global key, but a parametric one, this key is parameterized by an OCaml value, and not the raw key part.

So in the end, the only way to be used when accessing a contract balance is Storage.Contract.Balance.get, which takes a Contract_repr.t and gives a Tez_repr.t.

All these well-typed operations are generated by a set of functors, that come just before Storage in TEZOS_PROTOCOL.

### The *_storage modules¶

The two previous steps ensure that the ledger’s state is always accessed and updated in a well-typed way.

However, it does not enforce that, for instance, when a contract is deleted, all of the keys that store its state in the context are indeed deleted.

This last series of modules named *_storage is there to enforce just that kind of invariants: ensuring the internal consistency of the context structure.

These transaction do not go as far as checking that, for instance, when the destination of a transaction is credited, the source is also debited, as in some cases, it might not be the case.

## Above the Alpha_context¶

The three next sections describe the main entrypoints to the protocol: validation of blocks by the shell (that we often also call application), smart contracts, and RPC services.

The Main module is the entrypoint that’s used by the shell. It respects the module type that all protocol must follow. For that, its code is mostly plumbing,

### Starting from Apply¶

This is were you want to start on your first read. Even if some plumbing code is woven in, such as error cases declaration and registration, most of the proof-of-stake code has been written in a verbose style, to be understood with minimum OCaml knowledge.

You want to start from the shell entry points (validation of the block header, validation of an operation, finalization of a block validation), and follow the control flow until you hit the Alpha_context abstraction barrier. This will lead you to reading modules Baking and Amendment.

### Smart contracts¶

From Apply, you will also end up in modules Script_ir_translator and Script_interpreter. The former is the typechecker of Michelson that is called when creating a new smart contract, and the latter is the interpreter that is called when transferring tokens to a new smart contract.

### Protocol RPC API¶

Finally, the RPCs specific to Alpha are also defined above the Alpha_context barrier.

Services are defined in a few modules, divided by theme. Each module defines the RPC API: URL schemes with the types of parameters, and input and output JSON schemas. This interface serves three purposes. As it is thoroughly typed, it makes sure that the handlers (that are registered in the same file) have the right input and output types. It is also used by the client to perform RPC calls, to make sure that the URL schemes and JSON formats and consistent between the two parties. These two features are extremely useful when refactoring, as the OCaml typechecker will help us track the effects of an RPC API change on the whole codebase. The third purpose is of course, to make automatic documentation generation possible (as in tezos client rpc list/format). Each service is also accompanied by a caller function, that can be used from the client to perform the calls, and by the tests to simulate calls in a fake in-memory context.

It can be useful if you are a third party developer who wants to read the OCaml definition of the service hierarchy directly, instead of the automatically generated JSON hierarchy.