Raw HTTP - Clients

diggy · August 29, 2020, 3:29am

Warning Raw HTTP needs more chops to use than our language clients. We wrote this guide to help you build a Dgraph client in a new language.

It’s also possible to interact with Dgraph directly via its HTTP endpoints. This allows clients to be built for languages that don’t have access to a working gRPC implementation.

In the examples shown here, regular command line tools such as curl and jq are used. However, the real intention here is to show other programmers how they could implement a client in their language on top of the HTTP API. For an example of how to build a client on top of gRPC, refer to the implementation of the Go client.

Similar to the Go client example, we use a bank account transfer example.

Create the Client

A client built on top of the HTTP API will need to track three pieces of state for each transaction.

A start timestamp (start_ts). This uniquely identifies a transaction, and doesn’t change over the transaction lifecycle.
The set of keys modified by the transaction (keys). This aids in transaction conflict detection.

Every mutation would send back a new set of keys. The client must merge them with the existing set. Optionally, a client can de-dup these keys while merging.
The set of predicates modified by the transaction (preds). This aids in predicate move detection.

Every mutation would send back a new set of preds. The client must merge them with the existing set. Optionally, a client can de-dup these keys while merging.

Alter the database

The /alter endpoint is used to create or change the schema. Here, the predicate name is the name of an account. It’s indexed so that we can look up accounts based on their name.

$ curl -X POST localhost:8080/alter -d \
'name: string @index(term) .
type Person {
   name
}'

If all goes well, the response should be {"code":"Success","message":"Done"}.

Other operations can be performed via the /alter endpoint as well. A specific predicate or the entire database can be dropped.

To drop the predicate name:

$ curl -X POST localhost:8080/alter -d '{"drop_attr": "name"}'

To drop the type Film:

$ curl -X POST localhost:8080/alter -d '{"drop_op": "TYPE", "drop_value": "Film"}'

To drop all data and schema:

$ curl -X POST localhost:8080/alter -d '{"drop_all": true}'

To drop all data only (keep schema):

$ curl -X POST localhost:8080/alter -d '{"drop_op": "DATA"}'

Start a transaction

Assume some initial accounts with balances have been populated. We now want to transfer money from one account to the other. This is done in four steps:

Create a new transaction.
Inside the transaction, run a query to determine the current balances.
Perform a mutation to update the balances.
Commit the transaction.

Starting a transaction doesn’t require any interaction with Dgraph itself. Some state needs to be set up for the transaction to use. The start_ts can initially be set to 0. keys can start as an empty set.

For both query and mutation if the start_ts is provided as a path parameter, then the operation is performed as part of the ongoing transaction. Otherwise, a new transaction is initiated.

Run a query

To query the database, the /query endpoint is used. Remember to set the Content-Type header to application/graphql+- in order to ensure that the body of the request is correctly parsed.

To get the balances for both accounts:

$ curl -H "Content-Type: application/graphql+-" -X POST localhost:8080/query -d $'
{
  balances(func: anyofterms(name, "Alice Bob")) {
    uid
    name
    balance
  }
}' | jq

The result should look like this:

{
  "data": {
    "balances": [
      {
        "uid": "0x1",
        "name": "Alice",
        "balance": "100"
      },
      {
        "uid": "0x2",
        "name": "Bob",
        "balance": "70"
      }
    ]
  },
  "extensions": {
    "server_latency": {
      "parsing_ns": 70494,
      "processing_ns": 697140,
      "encoding_ns": 1560151
    },
    "txn": {
      "start_ts": 4,
    }
  }
}

Notice that along with the query result under the data field is additional data in the extensions -> txn field. This data will have to be tracked by the client.

For queries, there is a start_ts in the response. This start_ts will need to be used in all subsequent interactions with Dgraph for this transaction, and so should become part of the transaction state.

Run a Mutation

Now that we have the current balances, we need to send a mutation to Dgraph with the updated balances. If Bob transfers $10 to Alice, then the RDFs to send are:

<0x1> <balance> "110" .
<0x1> <dgraph.type> "Balance" .
<0x2> <balance> "60" .
<0x2> <dgraph.type> "Balance" .

Note that we have to refer to the Alice and Bob nodes by UID in the RDF format.

We now send the mutations via the /mutate endpoint. We need to provide our transaction start timestamp as a path parameter, so that Dgraph knows which transaction the mutation should be part of. We also need to set Content-Type header to application/rdf in order to specify that mutation is written in rdf format.

$ curl -H "Content-Type: application/rdf" -X POST localhost:8080/mutate?startTs=4 -d $'
{
  set {
    <0x1> <balance> "110" .
    <0x1> <dgraph.type> "Balance" .
    <0x2> <balance> "60" .
    <0x2> <dgraph.type> "Balance" .
  }
}
' | jq

The result:

{
  "data": {
    "code": "Success",
    "message": "Done",
    "uids": {}
  },
  "extensions": {
    "server_latency": {
      "parsing_ns": 50901,
      "processing_ns": 14631082
    },
    "txn": {
      "start_ts": 4,
      "keys": [
        "2ahy9oh4s9csc",
        "3ekeez23q5149"
      ],
      "preds": [
        "1-balance"
      ]
    }
  }
}

We get some keys. These should be added to the set of keys stored in the transaction state. We also get some preds, which should be added to the set of preds stored in the transaction state.

Committing the transaction

Note It’s possible to commit immediately after a mutation is made (without requiring to use the /commit endpoint as explained in this section). To do this, add the parameter commitNow in the URL /mutate?commitNow=true.

Finally, we can commit the transaction using the /commit endpoint. We need the start_ts we’ve been using for the transaction along with the keys and the preds. If we had performed multiple mutations in the transaction instead of just one, then the keys and preds provided during the commit would be the union of all keys and preds returned in the responses from the /mutate endpoint.

The preds field is used to abort the transaction in cases where some of the predicates are moved. This field is not required and the /commit endpoint also accepts the old format, which was a single array of keys.

$ curl -X POST localhost:8080/commit?startTs=4 -d $'
{
  "keys": [
		"2ahy9oh4s9csc",
		"3ekeez23q5149"
	],
  "preds": [
    "1-balance"
	]
}' | jq

The result:

{
  "data": {
    "code": "Success",
    "message": "Done"
  },
  "extensions": {
    "txn": {
      "start_ts": 4,
      "commit_ts": 5
    }
  }
}

The transaction is now complete.

If another client were to perform another transaction concurrently affecting the same keys, then it’s possible that the transaction would not be successful. This is indicated in the response when the commit is attempted.

{
  "errors": [
    {
      "code": "Error",
      "message": "Transaction has been aborted. Please retry."
    }
  ]
}

In this case, it should be up to the user of the client to decide if they wish to retry the transaction.

Aborting the transaction

To abort a transaction, use the same /commit endpoint with the abort=true parameter while specifying the startTs value for the transaction.

$ curl -X POST "localhost:8080/commit?startTs=4&abort=true" | jq

The result:

{
  "code": "Success",
  "message": "Done"
}

Running read-only queries

You can set the query parameter ro=true to /query to set it as a read-only query.

$ curl -H "Content-Type: application/graphql+-" -X POST "localhost:8080/query?ro=true" -d $'
{
  balances(func: anyofterms(name, "Alice Bob")) {
    uid
    name
    balance
  }
}

Running best-effort queries

You can set the query parameter be=true to /query to set it as a best-effort query.

$ curl -H "Content-Type: application/graphql+-" -X POST "localhost:8080/query?be=true" -d $'
{
  balances(func: anyofterms(name, "Alice Bob")) {
    uid
    name
    balance
  }
}

Compression via HTTP

Dgraph supports gzip-compressed requests to and from Dgraph Alphas for /query, /mutate, and /alter.

Compressed requests: To send compressed requests, set the HTTP request header Content-Encoding: gzip along with the gzip-compressed payload.

Compressed responses: To receive gzipped responses, set the HTTP request header Accept-Encoding: gzip and Alpha will return gzipped responses.

Example of a compressed request via curl:

$ curl -X POST \
  -H 'Content-Encoding: gzip' \
  -H "Content-Type: application/rdf" \
  localhost:8080/mutate?commitNow=true --data-binary @mutation.gz

Example of a compressed request via curl:

$ curl -X POST \
  -H 'Accept-Encoding: gzip' \
  -H "Content-Type: application/graphql+-" \
  localhost:8080/query -d $'schema {}' | gzip --decompress

Example of a compressed request and response via curl:

$ zcat query.gz # query.gz is gzipped compressed
{
  all(func: anyofterms(name, "Alice Bob")) {
    uid
    balance
  }
}

$ curl -X POST \
  -H 'Content-Encoding: gzip' \
  -H 'Accept-Encoding: gzip' \
  -H "Content-Type: application/graphql+-" \
  localhost:8080/query --data-binary @query.gz | gzip --decompress

Note

Curl has a --compressed option that automatically requests for a compressed response (Accept-Encoding header) and decompresses the compressed response.

$ curl -X POST --compressed -H "Content-Type: application/graphql+-" localhost:8080/query -d $'schema {}'

Run a query in JSON format

The HTTP API also accepts requests in JSON format. For queries you have the keys “query” and “variables”. The JSON format is required to set GraphQL Variables with the HTTP API.

This query:

{
  balances(func: anyofterms(name, "Alice Bob")) {
    uid
    name
    balance
  }
}

Should be escaped to this:

curl -H "Content-Type: application/json" localhost:8080/query -XPOST -d '{
    "query": "{\n balances(func: anyofterms(name, \"Alice Bob\")) {\n uid\n name\n balance\n }\n }"
}' | python -m json.tool | jq

This is a companion discussion topic for the original entry at https://dgraph.io/docs/clients/raw-http/

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Raw HTTP - Clients

Create the Client

Alter the database

Start a transaction

Run a query

Run a Mutation

Committing the transaction

Aborting the transaction

Running read-only queries

Running best-effort queries

Compression via HTTP

Run a query in JSON format

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