# Amazon SNS message replay for FIFO topic subscribers
Amazon SNS replay allows topic subscribers to retrieve and redeliver archived messages from the topic data store to a subscribed endpoint.
+ Messages can be replayed immediately after the subscription is created.
+ A replayed message retains the same content, `MessageId`, and `Timestamp` as the original.
+ The message includes a `Replayed` attribute to indicate that it is a replayed message.
+ To replay only specific messages, apply a filter policy to your subscription.
For more on filtering messages, see [Filter replayed messages](#message-archiving-and-replay-subscription-filtering).
## Create a message replay policy using the AWS Management Console
Use this option to create a new replay policy using the AWS Management Console.
1. Sign in to the [Amazon SNS console](https://console.aws.amazon.com/sns/home).
1. Choose a topic subscription or create a new one. To learn more about creating subscriptions, see [Creating a subscription to an Amazon SNS topic](sns-create-subscribe-endpoint-to-topic.md).
1. To initiate the message replay, go to the **Replay** drop-down and choose **Start replay**.
1. From the **Replay timeframe** modal, make the following selections:
1. **Choose replay start date and time** – Choose the **date** (YYYY/MM/DD format) and **time** (24-hour hh:mm:ss format) from which you want to start replaying archived messages. The start time should be later than the beginning of the approximated archive time.
1. **(Optional) Choose replay end date and time** – Choose the **date** (YYYY/MM/DD format) and **time** (24-hour hh:mm:ss format) when you want to stop replaying archived messages.
1. Choose **Start replay**.
1. (Optional) To **stop** a message replay, go to the **Subscription details** page and choose **Stop replay** from the **Replay** drop-down.
1. (Optional) To **monitor** message replay metrics from within this workflow using CloudWatch, see [Monitor message replay metrics using Amazon CloudWatch](#message-archiving-and-replay-subscription-cloudwatch).
**To view and edit a message replay policy**
You can perform the following actions from the **Subscription details** page:
+ To **view** the message replay status, the **Replay status** field displays the following values:
+ **Completed** – The replay has successfully redelivered all messages, and is now delivering newly published messages.
+ **In progress** – The replay is currently replaying the selected messages.
+ **Failed** – The replay was unable to complete.
+ **Pending** – The default state while the replay initiates.
+ (Optional) To **modify** a message replay policy, go to the **Subscription details** page and choose **Start replay** from the **Replay** drop-down. Starting a replay will replace the existing replay.
## Add a replay policy to the subscription using the API
To replay archived messages use the attribute `ReplayPolicy`. `ReplayPolicy` can be used with the `Subscribe` and `SetSubscriptionAttributes` API actions. This policy has the following values:
+ **`StartingPoint`** (Required) – Signals where to start replaying messages from.
+ **`EndingPoint`** (Optional) – Signals when to stop replaying messages. If `EndingPoint` is omitted, then the replay will continue until caught up to the current time.
+ **`PointType`** (Required) – Sets the type of starting and ending points. Currently, the supported value for `PointType` is `Timestamp`.
For example, to recover from a downstream failure and resend all messages for a two hour time period on October 1, 2023, use the `SetSubscriptionAttributes` API action to set a `ReplayPolicy` as follows:
```
{
"PointType":"Timestamp",
"StartingPoint":"2023-10-01T10:00:00.000Z",
"EndingPoint":"2023-10-01T12:00:00.000Z"
}
```
To replay all messages sent to the topic as of October 1, 2023, and continue receiving all newly published messages to your topic, use the `SetSubscriptionAttributes` API action to set a `ReplayPolicy` on your subscription as follows:
```
{
"PointType":"Timestamp",
"StartingPoint":"2023-10-01T00:00:00.000Z"
}
```
To verify that a message has been replayed, the boolean attribute `Replayed` is added to each replayed message.
## Add a replay policy to the subscription using the SDK
To use an AWS SDK, you must configure it with your credentials. For more information, see [Shared `config` and `credentials` files](https://docs.aws.amazon.com/sdkref/latest/guide/file-format.html) in the * AWS SDKs and Tools Reference Guide*.
The following code example shows how to set the `ReplayPolicy` on a subscription to redeliver messages from the Amazon SNS FIFO topic's archive for a 2-hour time window on October 1st 2023.
```
// Specify the ARN of the Amazon SNS subscription to initiate the ReplayPolicy on.
String subscriptionArn =
"arn:aws:sns:us-east-2:123456789012:MyArchiveTopic.fifo:1d2a3e9d-7f2f-447c-88ae-03f1c68294da";
// Set the ReplayPolicy to replay messages from the topic's archive
// for a 2 hour time period on October 1st 2023 between 10am and 12pm UTC.
String replayPolicy =
"{\"PointType\":\"Timestamp\",\"StartingPoint\":\"2023-10-01T10:00:00.000Z\",\"EndingPoint\":\"2023-10-01T12:00:00.000Z\"}";
// Set the ArchivePolicy for the Amazon SNS topic
SetSubscriptionAttributesRequest request = new SetSubscriptionAttributesRequest()
.withSubscriptionArn(subscriptionArn)
.withAttributeName("ReplayPolicy")
.withAttributeValue(replayPolicy);
sns.setSubscriptionAttributes(request);
```
## Understanding the EndingPoint
When you apply a `ReplayPolicy` to an Amazon SNS subscription, the `EndingPoint` value is optional. If no `EndingPoint` is provided, the replay will start from the specified `StartingPoint` and continue until it catches up to the current time, including processing any newly published messages. Once caught up, the subscription will function as a regular subscription, receiving new messages as they are published.
If an `EndingPoint` is specified, the service will replay messages from the `StartingPoint` up to the `EndingPoint` and then stop. **This action effectively pauses the subscription.** While the subscription is paused, newly published messages will not be delivered to the subscribed endpoint.
To resume message delivery, apply a new `ReplayPolicy` without providing an `EndingPoint`, and set the `StartingPoint` to the desired point in time from which to continue receiving messages. For example, to resume a subscription from where a prior replay finished, set the new `StartingPoint` to the previously provided `EndingPoint`.
## Filter replayed messages
Amazon SNS message filtering let's you control the replayed messages that Amazon SNS replays to your subscriber endpoint. When message filtering and message archiving are both enabled, Amazon SNS first retrieves the message from the topic’s data store, then applies the message against the subscription’s `FilterPolicy`. The message is delivered to the subscribed endpoint when there is a match, otherwise message is filtered out. For more information, see [Amazon SNS subscription filter policies](sns-subscription-filter-policies.md).
## Monitor message replay metrics using Amazon CloudWatch
You can monitor replay messages using Amazon CloudWatch using the following metrics. To be notified of anomalies in your workloads and help avoid impact, you can configure Amazon CloudWatch alarms on these metrics. For more details, see [Logging and monitoring in Amazon SNS](sns-logging-monitoring.md).
| Metric | Description |
| --- | --- |
| **NumberOfReplayedNotificationsDelivered** | Provides the subscriber with the aggregate number of messages replayed from the topic archive, at 1-minute resolution. |
| **NumberOfReplayedNotificationsFailed** | Provides the subscriber with the aggregate number of messages replayed that failed to deliver from the topic archive, at 1-minute resolution. |