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Upserts

Upsert-style operations are operations where:

  1. A node is searched for, and then
  2. Depending on if it is found or not, either:
    • Updating some of its attributes, or
    • Creating a new node with those attributes.

The upsert has to be an atomic operation such that either a new node is created, or an existing node is modified. It’s not allowed that two concurrent upserts both create a new node.

There are many examples where upserts are useful. Most examples involve the creation of a 1 to 1 mapping between two different entities. E.g. associating email addresses with user accounts.

Upserts are common in both traditional RDBMSs and newer NoSQL databases. Dgraph is no exception.

Upsert Procedure

In Dgraph, upsert-style behaviour can be implemented by users on top of transactions. The steps are as follows:

  1. Create a new transaction.

  2. Query for the node. This will usually be as simple as { q(func: eq(email, "[email protected]")) { uid }}. If a uid result is returned, then that’s the uid for the existing node. If no results are returned, then the user account doesn’t exist.

  3. In the case where the user account doesn’t exist, then a new node has to be created. This is done in the usual way by making a mutation (inside the transaction), e.g. the RDF _:newAccount <email> "[email protected]" .. The uid assigned can be accessed by looking up the blank node name newAccount in the Assigned object returned from the mutation.

  4. Now that you have the uid of the account (either new or existing), you can modify the account (using additional mutations) or perform queries on it in whichever way you wish.

Upsert Block

You can also use the Upsert Block to achieve the upsert procedure in a single mutation. The request will contain both the query and the mutation as explained here.

Conflicts

Upsert operations are intended to be run concurrently, as per the needs of the application. As such, it’s possible that two concurrently running operations could try to add the same node at the same time. For example, both try to add a user with the same email address. If they do, then one of the transactions will fail with an error indicating that the transaction was aborted.

If this happens, the transaction is rolled back and it’s up to the user’s application logic to retry the whole operation. The transaction has to be retried in its entirety, all the way from creating a new transaction.

The choice of index placed on the predicate is important for performance. Hash is almost always the best choice of index for equality checking.

Note It’s the index that typically causes upsert conflicts to occur. The index is stored as many key/value pairs, where each key is a combination of the predicate name and some function of the predicate value (e.g. its hash for the hash index). If two transactions modify the same key concurrently, then one will fail.