The Ledger API services

The Ledger API is structured as a set of services. The core services are implemented using gRPC and Protobuf, but most applications access this API through the mediation of the language bindings.

This page gives more detail about each of the services in the API, and will be relevant whichever way you’re accessing it.

If you want to read low-level detail about each service, see the protobuf documentation of the API.

Overview

The API is structured as two separate data streams:

  • A stream of commands TO the ledger that allow an application to submit transactions and change state.
  • A stream of transactions and corresponding events FROM the ledger that indicate all state changes that have taken place on the ledger.

Commands are the only way an application can cause the state of the ledger to change, and events are the only mechanism to read those changes.

For an application, the most important consequence of these architectural decisions and implementation is that the ledger API is asynchronous. This means:

  • The outcome of commands is only known some time after they are submitted.
  • The application must deal with successful and erroneous command completions separately from command submission.
  • Ledger state changes are indicated by events received asynchronously from the command submissions that cause them.

The need to handle these issues is a major determinant of application architecture. Understanding the consequences of the API characteristics is important for a successful application design.

For more help understanding these issues so you can build correct, performant and maintainable applications, read the application architecture guide.

Glossary

  • The ledger is a list of transactions. The transaction service returns these
  • A transaction is a tree of actions, also called events, which are of type create, exercise or archive. The transaction service can return the whole tree, or a flattened list.
  • A submission is a proposed transaction, consisting of a list of commands, which correspond to the top-level actions in that transaction.
  • A completion indicates the success or failure of a submission.

Submitting commands to the ledger

Command submission service

Use the command submission service to submit commands to the ledger. Commands either create a new contract instance, or exercise a choice on an existing contract.

A call to the command submission service will return as soon as the ledger server has parsed the command, and has either accepted or rejected it. This does not mean the command has been executed, only that the server has looked at the command and decided that its format is acceptable, or has rejected it for syntactic or content reasons.

The on-ledger effect of the command execution will be reported via the transaction service, described below. The completion status of the command is reported via the command completion service. Your application should receive completions, correlate them with command submission, and handle errors and failed commands. Alternatively, you can use the command service, which conveniently wraps the command submission and completion services.

Commands can be labeled with two application-specific IDs, both of which are returned in completion events:

  • A commandId, returned to the submitting application only. It is generally used to implement this correlation between commands and completions.
  • A workflowId, returned as part of the resulting transaction to all applications receiving it. It can be used to track workflows between parties, consisting of several transactions.

Command completion service

Use the command completion service to find out the completion status of commands you have submitted.

Completions contain the commandId of the completed command, and the completion status of the command. This status indicates failure or success, and your application should use it to update what it knows about commands in flight, and implement any application-specific error recovery.

Command service

Use the command service when you want to submit a command and wait for it to be executed. This service is similar to the command submission service, but also receives completions and waits until it knows whether or not the submitted command has completed. It returns the completion status of the command execution.

You can use either the command or command submission services to submit commands to effect a ledger change. The command service is useful for simple applications, as it handles a basic form of coordination between command submission and completion, correlating submissions with completions, and returning a success or failure status. This allow simple applications to be completely stateless, and alleviates the need for them to track command submissions.

Reading from the ledger

Transaction service

Use the transaction service to listen to changes in the ledger state, reported via a stream of transactions.

Transactions detail the changes on the ledger, and contains all the events (create, exercise, archive of contracts) that had an effect in that transaction.

Transactions contain a transactionId (assigned by the server), the workflowId, the commandId, and the events in the transaction.

Subscribe to the transaction service to read events from an arbitrary point on the ledger. This is important when starting or restarting and application, and to work in conjunction with the active contract service.

Transaction and transaction trees

TransactionService offers several different subscriptions. The most commonly used is GetTransactions. If you need more details, you can use GetTransactionTrees instead, which returns transactions as flattened trees, represented as a map of event IDs to events and a list of root event IDs.

Verbosity

The service works in a non-verbose mode by default, which means that some identifiers are omitted:

  • Record IDs
  • Record field labels
  • Variant IDs

You can get these included in requests related to Transactions by setting the verbose field in message GetTransactionsRequest or GetActiveContractsRequest to true.

Active contract service

Use the active contract service to obtain a party-specific view of all contracts currently active on the ledger.

The active contract service returns the current contract set as a set of created events that would re-create the state being reported. Each created event has a ledger offset where it occurs. You can infer the ledger offset of the contract set from the ledger offset of the last event you receive.

This is most important at application start, if the application needs to synchronize its initial state with a known view of the ledger. Without this service, the only way to do this would be to read the Transaction Stream from the beginning of the ledger, which can be prohibitively expensive with a large ledger.

Verbosity

See Verbosity above.

Utility services

Package service

Use the package service to obtain information about DAML packages available on the server.

This is useful for obtaining type and metadata information that allow you to interpret event data in a more useful way.

Ledger identity service

Use the ledger identity service to get the identity string of the ledger that it is connected to.

You need to include this identity string when submitting commands. Commands with an incorrect identity string are rejected.

Ledger configuration service

Use the ledger configuration service to subscribe to changes in ledger configuration.

This configuration includes maximum and minimum values for the difference in Ledger Effective Time and Maximum Record Time (see Time Service for details of these).

Testing services

These are only for use for testing with the Sandbox, not for on production ledgers.

Time service

Use the time service to obtain the time as known by the ledger server.

This is important because you have to include two timestamps when you submit a command - the Ledger Effective Time (LET), and the Maximum Record Time (MRT). For the command to be accepted, LET must be greater than the current ledger time.

MRT is used in the detection of lost commands.

Reset service

Use the reset service to reset the ledger state, as a quicker alternative to restarting the whole ledger application.

This resets all state in the ledger, including the ledger ID, so clients will have to re-fetch the ledger ID from the identity service after hitting this endpoint.

Services diagram

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