One of the most powerful features of AMPS is content filtering. With content filtering, filters based on message content are applied at the server so that your application and the network are not utilized by messages that are not relevant to your application. For example, if your application is only displaying messages from a particular user, you can send a content filter to the server so that only messages from that particular user are sent to the client.

To apply a content filter to a subscription, simply pass it into the Client.subscribe() call or use the filter() method to add a filter to the Command:

await client.subscribe(
  msg => console.log('Mom says: ', msg.data),
  'letters',         // Topic
  '/sender="mom"',   // Filter
  {timeout: 5000}    // Timeout (optional)
);

In this example, we have passed in a content filter /sender = 'mom'. This will result in the server only sending us messages, from the messages topic, that have the sender field equal to mom in the message.

For example, the AMPS server will send the following message, where /sender is mom:

{
    "sender" : "mom",
    "text" : "Happy Birthday!",
    "reminder" : "Call me Thursday!"
}

The AMPS server will not send a message with a different /sender value:

{
    "sender" : "henry dave",
    "text" : "Things do not change; we change."
}

AMPS allows you to update parameters, such as the content filter, on a subscription. When you replace a filter on the subscription, AMPS immediately begins sending only messages that match the updated filter. Notice that if the subscription was entered with a command that includes a SOW query, using the replace option can re-issue the SOW query (as described in the AMPS User Guide).

To update the filter on a subscription, you create a subscribe command. You set the SubscriptionId provided on the Command to the identifier of the existing subscription and include the replace option on the Command.

When you send the Command, AMPS atomically replaces the filter and sends messages that match the updated filter from that point forward.

await client.connect('wss://localhost:9000/amps/json');

// Original subscription
const subId = await client.execute(
  new Command('subscribe').topic('orders').filter('/id > 100'),
  onMessagePrinter
);

// replace the subscription with a new filter value
await client.execute(
  new Command('subscribe')
    .topic('orders')
    .filter('/id < 5')
    .subId(subId)
    .options('replace'),
  onMessagePrinter
);