# 5. Error Handling

In every distributed system, the robustness of your application depends on its ability to recover gracefully from unexpected events. The AMPS client provides the building blocks necessary to ensure your application can recover from the kinds of errors and special events that may occur when using AMPS.

## Exceptions

Generally speaking, when an error occurs that prohibits an operation from succeeding, AMPS will throw an exception. AMPS exceptions universally derive from `AMPS.AMPSException`, so by catching `AMPSException`, you will be sure to catch anything AMPS throws, for example:

    def read_and_evaluate(client):
        # read a new payload from the user
        payload = input("Please enter a message")

        # write a new message to AMPS
        if payload:
            try:
                client.publish(
                    "UserMessage",
                    "{ \"message\" : \"%s\"  }" % payload
                )
            except AMPS.AMPSException as e:
                sys.stderr.write("An AMPS exception" + "occurred: %s" % str(e))

Example 5.1: Catching an AMPS Exception

In this example, if an error occurs, the program writes the error to `stderr` and the `publish()` command fails. However, `client` is still usable for continued publishing and subscribing. When the error occurs, the exception is written to the console. As with most Python exceptions, `str()` will convert the exception into a string that includes a descriptive message.

AMPS exception types vary based on the nature of the error that occurs. In your program, if you would like to handle certain kinds of errors differently than others, you can handle the appropriate subclass of `AMPSException` to detect those specific errors and do something different.

    def create_new_subscription(client):
        messageStream = None
        topicName = None

        while messageStream is None:

        # attempts to retrieve a topic name (or regular expression) from the user.
        topicName = input("Please enter a topic name")
        try:

            # If an error occurs when setting up the subscription, the program decides whether
            # or not to try again based on the subclass of AMPSException that is thrown. In
            # this case, if the exception is a BadRegexTopicError, the exception indicates
            # that the user provided a bad regular expression. We would like to give the user
            # a chance to correct, so we ask the user for a new topic name.
            messageStream = client.subscribe(
                topicName,
                None
            )

        # This line indicates that the program catches the BadRegexTopicError exception
        # and displays a specific error to the user indicating the topic name or
        # expression was invalid. By not returning from the function in this except block,
        # the while loop runs again and the user is asked for another topic name.
        except BadRegexTopicError as e:
            print(
                "Error: bad topic name or regular expression " +
                topicName +
                ".  The exception was " +
                str(e) +
                "."
            )
            # we'll ask the user for another topic

        # If an AMPS exception of a type other than BadRegexTopicError is thrown by AMPS,
        # it is caught here. In that case, the program emits a different error message to
        # the user.
        except AMPSException as e:
            print (
                "Error: error setting up subscription to topic" +
                topicName +
                ".  The exception was " +
                str(e) +
                "."
            )

            # At this point the code stops attempting to subscribe to the client by the return
            # None statement.
            return None
       return messageStream

Example 5.2: Handling AMPSException Subclasses

### Exception Types

Each method in AMPS documents the kinds of exceptions that are thrown from it. For reference, Appendix A: Exceptions contains a list of all of the exception types you may encounter while using AMPS, when they occur, and what they mean.

### Exception Handling and Asynchronous Message Processing

When using asynchronous message processing, exceptions thrown from the message handler are silently absorbed by the AMPS Python client by default. The AMPS Python client allows you to register an exception listener to detect and respond to these exceptions. When an exception listener is registered, AMPS will call the exception listener with the exception. See Example 5.5 for details.

## Controlling Blocking with Command Timeout

The named convenience methods and the `Command` class provide a `timeout` setting that specifies how long the command should wait to receive a `processed` acknowledgment from AMPS. This can be helpful in cases where it is important for the caller to limit the amount of time to block waiting for AMPS to acknowledge the command. If the AMPS client does not receive the processed acknowledgment within the specified time, the client sends an `unsubscribe` command to the server to cancel the command and throws an exception.

Acknowledgments from AMPS are processed by the client receive thread on the same socket as data from AMPS. This means that any other data previously returned (such as the results of a large query) must be consumed before the acknowledgment can be processed. An application that submits a set of SOW queries in rapid succession should set a timeout that takes into account the amount of time required to process the results of the previous query.

## Disconnect Handling

Every distributed system will experience occasional disconnections between one or more nodes. The reliability of the overall system depends on an application’s ability to efficiently detect and recover from these disconnections. Using the AMPS Python client’s disconnect handling, you can build powerful applications that are resilient in the face of connection failures and spurious disconnects. For additional reliability, you can also use the high availability client (discussed in the following sections), which provides both disconnect handling and features to help ensure that messages are reliably delivered.

### Using a Heartbeat to Detect Disconnection

The AMPS client includes a heartbeat feature to help applications detect disconnection from the server within a predictable amount of time. Without using a heartbeat, an application must rely on the operating system to notify it when a disconnect occurs. For applications that are simply receiving messages, it can be impossible to tell whether a socket is disconnected or whether there are simply no incoming messages for the client.

When you set a heartbeat, the AMPS client sends a heartbeat message to the AMPS server at a regular interval, and waits a specified amount of time for the response. If the operating system reports an error on send, or if there is no message received from the server within the specified amount of time, the AMPS client considers the server to be disconnected. Likewise, the server will ensure that traffic is sent to the client at the specified interval, using heartbeat messages when no other traffic is being sent to the client. If, after sending a heartbeat message, no traffic from the client arrives within a period twice the specified interval, the server will consider the client to be disconnected or nonresponsive.

The AMPS client processes heartbeat messages on the client receive thread, which is the thread used for asynchronous message processing. If your application uses asynchronous message processing and occupies the thread for longer than the heartbeat interval, the client may fail to respond to heartbeat messages in a timely manner and may be disconnected by the server.

### Managing Disconnection

The `HAClient` class, included with the AMPS Python client, contains a disconnect handler and other features for building highly-available applications. The `HAClient` includes features for managing a list of failover servers, resuming subscriptions, republishing in-flight messages, and other functionality that is commonly needed for high availability. 60East recommends using the `HAClient` for automatic reconnection wherever possible, as the HAClient disconnect handler has been carefully crafted to handle a wide variety of edge cases and potential failures.

If an application needs to reconnect or fail over, use an `HAClient`, and the AMPS client library will automatically handle failover and reconnection. You control which servers the client fails over to using an implementation of the `ServerChooser` interface, and you can control the timing of the failover using an implementation of the `ReconnectDelayStrategy` interface.

Tip

For most applications, the combination of the `HAClient` disconnect handler and a `ConnectionStateListener` gives you the ability to monitor disconnections and add custom behavior at the appropriate point in the reconnection process.

If you need to add custom behavior to the failover (such as logging, resetting an internal cache, refreshing credentials and so on), the `ConnectionStateListener` class allows your application to be notified and take action when disconnection is detected and at each stage of the reconnection process.

To extend the behavior of the AMPS client during reconnection, implement a `ConnectionStateListener`.

### Replacing Disconnect Handling

In some cases, an application does not want the AMPS client to reconnect, but instead wants to take a different action if disconnection occurs. For example, a stateless publisher that sends ephemeral data (such as telemetry or prices) may want to exit with an error if the connection is lost rather than risk falling behind and providing outdated messages. Often, in this case, a monitoring process will start another publisher if a publisher fails, and it is better for a message to be lost than to arrive late.

To cover cases where the application has unusual needs, the AMPS client library allows an application to provide custom disconnect handling.

Your application gets to specify exactly what happens when a disconnect occurs by supplying a function to `client.set_disconnect_handler()`, which is invoked whenever a disconnect occurs. This may be helpful for situations where a particular connection needs to do something completely different than reconnecting or failing over to another AMPS server.

Caution

Setting the disconnect handler completely replaces the disconnection and failover behavior for an `HAClient` and provides the only disconnection and failover behavior for a `Client`.

The handler runs on the thread that detects the disconnect. This may be the client receive thread (for example, if the disconnect is detected due to heartbeating) or an application thread (for example, if the disconnect is detected when sending a command to AMPS).

The example below shows the basics:

    class MyApp:
        def __init__(self, _uri):
            self.uri = _uri
            self.client = None
            self.client = AMPS.Client(...)

            # set_disconnect_handler() method is called to supply a function for use when AMPS
            # detects a disconnect. At any time, this function may be called by AMPS to
            # indicate that the client has disconnected from the server, and to allow your
            # application to choose what to do about it. The application continues on to
            # connect and subscribe to the orders topic.
            self.client.set_disconnect_handler(self.exit_on_disconnection)
            self.client.connect(self.uri)
            self.client.logon()

        # display order data to the user
        def showMessage(self,m):
            pass

        # Our disconnect handler’s implementation begins here.
        #
        # In this example, we exit the application if the
        # connection fails.

        def exit_on_disconnection(self, client):
            sys.exit(1)

Example 5.3: Supplying a Disconnect Handler

## Unexpected Messages

The AMPS Python client handles most incoming messages and takes appropriate action. Some messages are unexpected or occur only in very rare circumstances. The AMPS Python client provides a way for clients to process these messages. Rather than providing handlers for all of these unusual events, AMPS provides a single handler function for messages that can’t be handled during normal processing.

Your application registers this handler by setting the `last_chance_message_handler` for the client. This handler is called when the client receives a message that can’t be processed by any other handler. This is a rare event, and typically indicates an unexpected condition.

For example, if a client publishes a message that AMPS cannot parse, AMPS returns a failure acknowledgment. This is an unexpected event, so AMPS does not include an explicit handler for this event, and failure acknowledgments are received in the method registered as the `last_chance_message_handler`.

Your application is responsible for taking any corrective action needed. For example, if a message publication fails, your application can decide to republish the message, publish a compensating message, log the error, stop publication altogether, or any other action that is appropriate.

## Unhandled Exceptions

When using the asynchronous interface, exceptions can occur that are not thrown to the user. For example, when an exception occurs in the process of reading subscription data from the AMPS server, the exception occurs on a thread inside of the AMPS Python client. Consider the following example using the asynchronous interface:

    class MyApp:
        def on_message_handler(self,message):
            print(message.get_data())

        def wait_to_be_poked(self,client):
            client.subscribe(
                self.on_message_handler,
                "pokes",
                "/Pokee LIKE '%s'" % getpass.getuser(),
                timeout=5000)
            input("Press enter to exit")

Example 5.4: Where do Exceptions go?

In this example, we set up a subscription to wait for messages on the pokes topic, whose Pokee tag begins with our user name. When messages arrive, we print a message out to the console, but otherwise our application waits for a key to be pressed.

Inside of the AMPS client, the client creates a new thread of execution that reads data from the server, and invokes message handlers and disconnect handlers when those events occur. When exceptions occur inside this thread, however, there is no caller for them to be thrown to and by default they are ignored.

In applications that use the asynchronous interface, and where it is important to deal with every issue that occurs in using AMPS, you can set an `ExceptionHandler` via `Client.set_exception_listener()` that receives these otherwise unhandled exceptions. Making the modifications shown in the example below, to our previous example, will allow those exceptions to be caught and handled. In this case we are simply printing those caught exceptions out to the console.

Tip

In some cases, the AMPS Python client may wrap exceptions of unknown type into an `AMPSException`. Your application should always include an except block for `AMPSException`.

If your application will attempt to recover from an exception thrown on the background processing thread, your application should set a flag and attempt recovery on a different thread than the thread that called the exception listener.

Tip

At the point that the AMPS client calls the exception listener, it has handled the exception. Your exception listener must not rethrow the exception (or wrap the exception and throw a different exception type).

    class MyApp:
        def on_exception(self, e):
            print ("Exception occurred: %s" % str(e))

        def on_message_handler(self,message):
            print (message.get_data())

        def wait_to_be_poked(self, client):
            client.set_exception_listener(self.on_exception)

            # Use the advanced interface to be able to
            # accept input while processing messages.

            client.subscribe(
                self.on_message_handler,
                "pokes",
                "/Pokee LIKE '%s'" % getpass.getuser(),
                timeout=5000)
            input("Press enter to exit")

Example 5.5: Exception Listener

In this example we have added a call to `client.set_exception_listener()`, registering a simple function that writes the text of the exception out to the console. If exceptions are thrown in the message handler, those exceptions are written to the console.

AMPS records the stack trace and provides it to the exception handler, if the provided method includes a parameter for the stack trace. The sample below demonstrates one way to do this. (For sample purposes, the message handler always throws an exception.)

    import AMPS
    import time
    import traceback

    def handler(message):
        print (message)
        raise RuntimeError("in my handler")

    def exception_listener(exception, tb):
        print ("EXCEPTION RECEIVED", exception)
        if tb is not None:
            traceback.print_tb(tb)

    client = AMPS.Client("client")

    client.set_exception_listener(exception_listener)

    client.connect("tcp://localhost:9007/amps/json")

    client.logon()
    client.subscribe(handler,"topic")
    client.publish("topic","data")
    time.sleep(1)
    client.close()

Example 5.6: AMPS stack trace

## Detecting Write Failures

The `publish` methods in the Python client deliver the message to be published to AMPS then return immediately, without waiting for AMPS to return an acknowledgment. Likewise, the `sow_delete` methods request deletion of SOW messages, and return before AMPS processes the message and performs the deletion. This approach provides high performance for operations that are unlikely to fail in production. However, this means that the methods return before AMPS has processed the command, without the ability to return an error in the event the command fails.

The AMPS Python client provides a `failed_write_handler` that is called when the client receives an acknowledgment that indicates a failure to persist data within AMPS. As with the `last_chance_message_handler` described in the Unexpected Messages section, your application registers a handler for this function. When an acknowledgment returns that indicates a failed write, AMPS calls the registered handler method with information from the acknowledgment message, supplemented with information from the client publish store if one is available. Your client can log this information, present an error to the user or take whatever action is appropriate for the failure.

If your application needs to know whether publishes succeeded and are durably persisted, the following approach is recommended:

- Set a `PublishStore` on the client. This will ensure that messages are retransmitted if the client becomes disconnected before the message is acknowledged and request `persisted` acknowledgments for messages.
- Install a `failed_write_handler`. In the event that AMPS reports an error for a given message, that event will be reported to the `failed_write_handler`.
- Call `publish_flush()` and verify that all messages are persisted before the application exits.

When no `failed_write_handler` is registered, acknowledgments that indicate errors in persisting data are treated as unexpected messages and routed to the `last_chance_message_handler`. In this case, AMPS provides only the acknowledgment message and does not provide the additional information from the client publish store.

## Monitoring Connection State

The AMPS client interface provides the ability to set one or more connection state listeners. A connection state listener is a callback that is invoked when the AMPS client detects a change to the connection state.

A connection state listener may be called from the client receive thread. An application should not submit commands to AMPS from a connection state listener, or the application risks creating a deadlock for commands that wait for acknowledgment from the server.

The AMPS client provides the following state values for a connection state listener:

State | Indicates
--- | ---
Connected | The client has established a connection to AMPS. If you are using a `Client`, this is delivered when `connect()` is successful. If you are using an `HAClient`, this state indicates that the `connect` part of the connect and logon process has completed. An `HAClient` using the default disconnect handler will attempt to log on immediately after delivering this state. Most applications that use `Client` will attempt to log on immediately after the call to `connect()` returns. An application should not submit commands to AMPS from the connection state listener while the client is in this state unless the application knows that the state has been delivered from a `Client` and that the `Client` does not call `logon()`.
LoggedOn | The client has successfully logged on to AMPS. If you are using a `Client`, this is delivered when `logon()` is successful. If you are using an `HAClient`, this state indicates that the `logon` part of the connect and logon process has completed. This state is delivered after the client is logged on, but before recovery of client state is complete. Recovery will continue after delivering this state: the application should not submit commands to AMPS from the connection state listener while the client is in this state if further recovery will take place.
HeartbeatInitiated | The client has successfully started heartbeat monitoring with AMPS. This state is delivered if the application has enabled heartbeating on the client. This state is delivered before recovery of the client state is complete. Recovery may continue after this state is delivered. The application should not submit commands to AMPS from the connection state listener until the client is completely recovered.
PublishReplayed | Delivered when a client has completed replay of the publish store when recovering after connecting to AMPS. This state is delivered when the client has a PublishStore configured. If the client has a subscription manager set, (which is the default for an `HAClient`), the application should not submit commands from the connection state listener until the `Resubscribed` state is received.
Resubscribed | Delivered when a client has re-entered subscriptions when recovering after connecting to AMPS. This state is delivered when the client has a subscription manager set (which is the default for an `HAClient`). This is the final recovery step. An application can submit commands to AMPS from the connection state listener after receiving this state.
Disconnected | The client is not connected. For an `HAClient`, this means that the client will attempt to reconnect to AMPS. For a `Client`, this means that the client will invoke the disconnect handler, if one is specified.
Shutdown | The client is shut down. For an `HAClient`, this means that the client will no longer attempt to reconnect to AMPS. This state is delivered when `close()` is called on the client or when a `ServerChooser` tells the `HAClient` to stop reconnecting to AMPS.

Table 5.1: ConnectionStateListener values

The enumeration provided for the connection state listener also includes a value of `UNKNOWN` for use as a default or to represent additional states in a custom `Client` implementation. The 60East implementations of the client do not deliver this state.

The following table shows examples of the set of states that will be delivered during connection, in order, depending on what features of the client are set. Notice that, for an instance of the `Client` class, this table assumes that the application calls both `connect()` and `logon()`. For an `HAClient`, this table assumes that the `HAClient` is using the default `DisconnectHandler` for the `HAClient`.

Configuration | States
--- | ---
Subscription Manager Publish Store | Connected LoggedOn PublishReplayed Resubscribed
Subscription Manager Publish Store Heartbeat Set | Connected LoggedOn HeartbeatInitiated PublishReplayed Resubscribed
Subscription Manager | Connected LoggedOn Resubscribed
Subscription Manager Heartbeat Set | Connected LoggedOn HeartbeatInitiated Resubscribed
(Default `Client` Configuration) | Connected LoggedOn

Table 5.2: Sequence of states for connection