Snapshot Server Operators

This document is intended as a guide for understanding and using the Snapshot Server operators.


The Snapshot Server is the operator which houses the most recent snapshot of tabular data. The Snapshot Server receives queries from an embedded query operator and sends query results to a result operator. The most recent tabular data can be sent in two forms: a list of maps and a list of POJOs. We will now describe how to visualize both forms of tabular data.


The AppDataSnapshotServerMap operator accepts a list of maps. Each map represents a row of a table. Each key in the map represents a column of the table and each value represents the value of a column for a particular row. The name, and type of each column is specified through the snapshotSchemaJSON property. The following is an example of a snapshotSchema:

  "values": [
    {"name": "word", "type": "string"}, 
    {"name": "count", "type": "integer"}

Here you can see that the table has two columns word and count of types string and integer respectively. According to this schema it is expected that the Maps in the provided list of Maps will have values for the keys “word” and “count”.

Now that the Snapshot server is configured we need to connect the embedded query operator and the result operator. The code snippet below shows how to do this:

String gatewayAddress = dag.getValue(DAG.GATEWAY_CONNECT_ADDRESS);
URI uri = URI.create("ws://" + gatewayAddress + "/pubsub");

AppDataSnapshotServerMap snapshotServer = dag.addOperator("SnapshotServer", new AppDataSnapshotServerMap());

PubSubWebSocketAppDataQuery wsQuery = new PubSubWebSocketAppDataQuery();

PubSubWebSocketAppDataResult wsResult = dag.addOperator("QueryResult", new PubSubWebSocketAppDataResult());

dag.addStream("Result", snapshotServer.queryResult, queryResultPort);

Note that the dt.attr.GATEWAY_CONNECT_ADDRESS property needs to be specified in the dt-site.xml and must contain the gateway’s ip address. This value will be automatically set if the application is launched through dtGateway.


The AppDataSnapshotServerPOJO operator accepts a list of POJOs. Each POJO represents a row of a table. Each field of the POJO represents represents the value for a particular column. The AppDataSnapshotServerPOJO is configured in the same way as the AppDataSnapshotServerMap except that there is an addition property that must be set. This additional property is fieldToGetter property. The fieldToGetter property is a Map from String to String. The key is the name of a column as defined in the SchemaJSON string. The value is the java getter method that is required to extract the value of that column. Let’s walk through an example of a valid setting for fieldToGetter. If this schema is configured:

  "values": [
    {"name": "word", "type": "string"}, 
    {"name": "count", "type": "integer"}

A valid setting for fieldToGetter would be this:

fieldToGetter.put(“word”, “getWord()”);
fieldToGetter.put(“count”, “getCount()”);


Please look at the following java code for example usages of the operators mentioned above: