Cassandra contexts

In any Cassandra filter, you can always depend on the following objects being defined.

Keep in mind that JavaScript objects should not be stored beyond the invocation of a filter, except in the thread context (see JavaScript environment for details). If you need to store an object beyond the invocation of a filter, use context.utils.createObject().

Invocation context (context)

This is the context in which a filter gets invoked. Any variables or functions you define in this context will be thrown away once the filter returns.

Pre-defined variables

• packet: the packet or token being filtered. The exact nature of this packet depends on the filter's circumstances. You can always get the type of the packet using the packetType property.

• utils: an object with a few useful methods

• filterContext: see below

• threadContext: see below

• connectionContext: see below

• projectContext: see below

Example

let pkt = context.packet;

if (pkt.packetType === "Query") {

    pkt.query += " limit 50";

}

Filter context (context.filterContext)

This context is shared by all invocations of a given filter.

Pre-defined variables

There may be one or more variable defined in this context with a name that starts with an underscore. These are internal variables and should not be used.

You can define additional variables to store data that would be useful to subsequent invocations of this filter.

Example

let startTime = context.filterContext.startTime;
if ( ! startTime) {
  startTime =  (new Date()).getTime();
  context.filterContext.startTime = startTime;
}

Connection context (context.connectionContext)

This context is shared by all filters invoked during the lifetime of the connection from the database client.

Pre-defined variables

• clientIP: the IP address of the client, e.g. /23.45.67.89 or /2600:1701:6920:69a0:17:986a:da7a:c12a

• userName: the name of the database user, if known.

• currentQuery: the last SQL to be executed on this connection, for the stream of the current packet. Gallium Data remembers open prepared statements and sets this value to the proper SQL for Prepare and Query packets.

Example

let sql = context.connectionContext.currentQuery;
if ( ! sql.toLowerCase().startsWith("select")) {
    log.debug("Non-select SQL statement: " + sql);
}

Project context (context.projectContext)

This context is as close as you can get to "global" variables. It is shared by all filters within a project, as long as the instance of Gallium Data is running.

Pre-defined variables

None

Thread context (context.threadContext)

This is a special context that is only shared by invocations of various filters that happen to be on the same Java thread. Anything stored in this context is thrown away when the thread is terminated, which can happen at any time between filter invocations. This context can be useful when dealing with non-thread-safe objects.

Pre-defined variables

None

Example

let startDate = context.threadContext.startDate;
if ( ! startDate) {
    startDate = new Date();
    // Note that we are storing a JavaScript object (Date), which is OK only in threadContext.
    context.threadContext.startDate = startDate;
}