chrisqwang3 commented on code in PR #24625:
URL: https://github.com/apache/pulsar/pull/24625#discussion_r2288989546
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pip/pip-437.md:
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+# PIP-437: Granular and Fixed-Delay Policies for Message Delivery
+
+# Background
+Pulsar's delayed delivery feature allows producers to schedule messages to be
delivered at a future time. To provide administrative control and prevent
abuse, **PIP-315** introduced a `maxDeliveryDelayInMillis` policy.
+While this provides an important safeguard by setting an *upper bound*, it
does not address all administrative needs. There is currently no way to enforce
a *specific, non-overridable* delay for all messages on a topic.
+
+# Motivation
+The current system lacks administrative controls on message delivery delays,
which introduces risks to cluster stability and data integrity.
+
+## Risks in the current system:
+Unbounded Resource Pressure: Excessively delayed messages must be retained in
storage for extended periods.
+More importantly, they create a long-lived state in the broker's memory that
cannot be cleared,
+leading to unpredictable resource consumption and potential garbage collection
pressure
+
+### The Limitation:
+To protect broker memory, this tracker is capped at a configurable fixed
number of entries (e.g., 10,000).
+A large volume of messages with widely varying delivery times can easily
exhaust this capacity.
+Once the tracker is full, it stops persisting the index for new delayed
messages, retaining this state only in memory.
+If the broker restarts for any reason (e.g., crash, rolling upgrade), this
volatile in-memory state is completely lost.
+
+### The Impact: Upon restart, the broker's only source of truth is the last
persisted cursor position.
+This position does not account for the lost tracking information, forcing the
broker to re-dispatch messages that were already processed.
+This results in significant and difficult-to-predict message duplication for
downstream consumers.
+
+The lack of administrative controls on message delivery delays introduces
critical risks to cluster stability and data integrity.
+This proposal aims to provide granular control at the topic and namespace
levels to add a new fixed delay delivery configuration:
+
+1. **Prevent Message Duplication:** By forcing all messages on a topic to
have the exact same delay,it prevents the delayed message tracker from becoming
overwhelmed and eliminates the risk of message duplication upon broker restart,
leading to a more stable and predictable system.
+2. **Enforcing Compliance and Business Rules:** Certain workflows may require
certain control that require the enforcement at the broker level
+3. **Simplifying Producer Configuration:** By setting a fixed delay on the
topic, the responsibility for managing the delay logic is shifted from the
client to the broker, reducing the risk of client-side misconfiguration.
+
+# Goals
+## In Scope
+- Introduce a new `fixed-delivery-delay` policy, configurable at the
namespace and topic levels, to enforce a mandatory delivery delay.
+- Enhance the existing `/delayedDelivery` admin API endpoints and
`pulsar-admin` commands to manage both the existing `maxDeliveryDelayInMillis`
and the new `fixedDeliveryDelayInMillis` policies within the same policy group.
+- Ensure the `fixed-delivery-delay` policy takes precedence over any
client-specified delay and the existing `max-delivery-delay` policy.
+
+# High Level Design
+This proposal will enhance the existing `DelayedDeliveryPolicies` object to
include a new `fixedDeliveryDelayInMillis` field.
+This new policy will follow Pulsar's standard hierarchical model, allowing it
to be set at the namespace level and overridden at the topic level.
+The core logic will be implemented with the following precedence:
+
+1. **If `fixed-delivery-delay` is set:** The broker will ignore any
`deliverAt` time sent by the producer and will override it by calculating
`publish_time + fixed_delay`. The `max-delivery-delay` policy is ignored.
+2. **If `fixed-delivery-delay` is NOT set, but `max-delivery-delay` is:** The
broker will validate the producer's requested `deliverAt` time against the
`max-delivery-delay` policy, rejecting the message if it exceeds the limit.
+3. **If neither policy is set:** The system behaves as it does today,
honoring the client's requested `deliverAt` time, constrained only by the
global broker-level setting.
+
+# Detailed Design
+## Design & Implementation Details
+
+1. **Data Model Changes**:
+ * Add a new `fixedDeliveryDelayInMillis` field to the
`DelayedDeliveryPolicies.java` class. This class is already used for the
existing `maxDeliveryDelayInMillis` policy.
+ * Update `HierarchyTopicPolicies.java` to resolve the effective
`fixedDeliveryDelayInMillis` for a topic, respecting the topic-over-namespace
hierarchy.
+
+2. **Enforcement Logic**:
+ * The enforcement logic will be placed in `PersistentTopic.java` within
the `publishMessage` and `publishTxnMessage` methods, right before the existing
`isExceedMaximumDeliveryDelay` check.
+ * This logic will modify the `MessageMetadata` of the incoming message
*before* it is passed to the managed ledger for persistence.
+
+## Public-facing Changes
+### Public API
+#### Topic-Level Policies
+| Method | Endpoint
| Description
|
+| :------- |
:------------------------------------------------------------------------
|:-----------------------------------------------------------------------------------------------------------------------------------------|
+| `POST` |
`/admin/v2/persistent/{tenant}/{namespace}/{topic}/delayedDelivery` |
Sets or updates the delayed delivery policies for the topic.To disable a
policy, a field can be set to `0`. |
+| `GET` |
`/admin/v2/persistent/{tenant}/{namespace}/{topic}/delayedDelivery` |
Gets the configured delayed delivery policies for the topic which optionally
include the fixedDeliveryDelayInMillis if configured |
+
+#### Namespace-Level Policies
+| Method | Endpoint
| Description |
+| :------- | :----------------------------------------------------------------
|:-----------------------------------------------------------------|
+| `POST` | `/admin/v2/namespaces/{tenant}/{namespace}/delayedDelivery`
| Sets or updates the delayed delivery policies for the namespace. To disable a
policy, a field can be set to `0`.|
+| `GET` | `/admin/v2/namespaces/{tenant}/{namespace}/delayedDelivery`
| Gets the configured delayed delivery policies for the namespace which
optionally include the fixedDeliveryDelayInMillis if configured |
+
+### Binary protocol
+
+### Configuration
+
+### CLI
+* The existing `set-delayed-delivery` command in `CmdTopicPolicies.java` and
`CmdNamespaces.java` will be updated with a new optional parameter:
`--fixed-delay` (or `-fd`).
+* The REST endpoints under `/delayedDelivery` will be updated to accept and
return the new `fixedDeliveryDelayInMillis` field in their JSON payload.
+
+### Metrics
+
+To provide visibility into the enforcement of the new policies, we will
introduce a new counter metric.
+* **Full Name**: `pulsar.broker.topic.messages.delayed.rejected`
Review Comment:
updated
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