---
title: Overview
weight: 1
type: docs
---
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## Introduction

Memory is a system that remembers information about previous interactions. Memory has emerged, and will continue to remain, a core capability of foundation model-based agents.
It underpins long-horizon reasoning, continual adaptation, and effective interaction with complex
environments.<sup>[1](https://arxiv.org/pdf/2512.13564)</sup>

We propose a distributed memory system for flink-agents that mirrors human cognitive processes. The solution implements four memory types: Sensory Memory, Short-term Memory, Long-term Memory and Knowledge, based on their visibility, retention and derivation.

## Memory Types

Flink Agents classifies memory types based on visibility, retention and derivation.

* **visibility**: The recall scope of the memory, i.e., whether it is visible across keys.
  * In Flink Agents, inputs of the agent are partitioned by their keys. This corresponds to how data are partitioned by keys in Flink's Keyed DataStream.
* **retention**: How long the memory will be remembered, within a single run or across multiple runs.
  * An agent run refers to a complete execution of an agent to process an input event. Each record from upstream will trigger a new agent run.
* **derivation**: Whether the retrieved information can be a derived version of the original one.
  * original: The retrieved information is exactly the same as how it was stored.
  * derived: The retrieved information might be derived from the original information.

{{< img src="/fig/memory/memory_types.png" alt="Memory Types" >}}

### Sensory Memory
* **Use Case**: Store temporary data generated during the agent execution which is only needed for a single run. For example, tool call context, or the data users want to pass to other agent actions.
* **Characteristics**: Isolation between agent runs, which means the memory will be automatically cleaned up by the framework when an agent run is completed.

For more details, see [Sensory & Short-term Memory]({{< ref "docs/development/memory/sensory_and_short_term_memory" >}}).

### Short-Term Memory
* **Use Case**: Store data generated during the agent execution. 
  * Compared to sensory memory, the lifecycle of stored data can across multiple runs.
  * Compared to long-term memory, user need retrieve all the original data they have written to the memory.
* **Characteristics**: 
  * The lifecycle of the stored data can across multiple runs. 
  * Complete original data retrieval.
  * Supports optional automatic expiration and cleanup.

For more details, see [Sensory & Short-Term Memory]({{< ref "docs/development/memory/sensory_and_short_term_memory" >}}).

### Long-Term Memory
* **Use Case**: Store data generated during the agent execution, but compared to short-term memory,
the data may expand rapidly as the agent execution, which requires compaction to provide concise and highly related context.
* **Characteristics**: 
  * Support for compacting similar items.
  * Support for semantic search.

For more details, see [Long-Term Memory]({{< ref "docs/development/memory/long_term_memory" >}}).

## Comparison

| Feature               | Sensory Memory                             | Short-Term Memory                             | Long-Term Memory                              |
|-----------------------|--------------------------------------------|-----------------------------------------------|-----------------------------------------------|
| **Retention**         | Single Run                                 | Multiple Runs                                 | Multiple Runs                                 |
| **Visibility**        | Single-Key                                 | Single-Key                                    | Single-Key                                    |
| **Derivation**        | Original                                   | Original                                      | Derived                                       |
| **Storage**           | Flink State                                | Flink State                                   | External Vector Store                         |
| **Access Pattern**    | Key-Value / Nested Objects                 | Key-Value / Nested Objects                    | Semantic Search                               |
| **Use Case**          | Temporary execution states of an agent run | Exact information shared across multiple runs | Large, compactable and searchable information |
| **Types of Contents** | Any primitive/object                       | Any primitive/object                          | String, ChatMessage                           |