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Overview

Implementation

Clients can communicate with the server in two different ways:

  • Via gRPC: internally this uses Protocol Buffers (the proto file used by Dgraph is located at api.proto).

  • Via HTTP: there are various endpoints, each accepting and returning JSON. There is a one to one correspondence between the HTTP endpoints and the gRPC service methods.

It’s possible to interface with Dgraph directly via gRPC or HTTP. However, if a client library exists for you language, this will be an easier option.

Tip For multi-node setups, predicates are assigned to the group that first sees that predicate. Dgraph also automatically moves predicate data to different groups in order to balance predicate distribution. This occurs automatically every 10 minutes. It’s possible for clients to aid this process by communicating with all Dgraph instances. For the Go client, this means passing in one *grpc.ClientConn per Dgraph instance. Mutations will be made in a round robin fashion, resulting in an initially semi random predicate distribution.

Transactions

Dgraph clients perform mutations and queries using transactions. A transaction bounds a sequence of queries and mutations that are committed by Dgraph as a single unit: that is, on commit, either all the changes are accepted by Dgraph or none are.

A transaction always sees the database state at the moment it began, plus any changes it makes — changes from concurrent transactions aren’t visible.

On commit, Dgraph will abort a transaction, rather than committing changes, when a conflicting, concurrently running transaction has already been committed. Two transactions conflict when both transactions:

  • write values to the same scalar predicate of the same node (e.g both attempting to set a particular node’s address predicate); or
  • write to a singular uid predicate of the same node (changes to [uid] predicates can be concurrently written); or
  • write a value that conflicts on an index for a predicate with @upsert set in the schema (see upserts).

When a transaction is aborted, all its changes are discarded. Transactions can be manually aborted.

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