Pulsar Seek lets you jump to a specific message by its ID or timestamp, but the real magic is how it fundamentally alters your read latency assumptions.
Let’s see it in action. Imagine you have a Pulsar topic persistent://public/default/my-topic and you’ve produced messages into it. Normally, you’d consume them in strict order from the earliest available. But with seek, you can instantly jump to a point in history.
Here’s a Python consumer using seek to jump to a specific message ID:
import pulsar
client = pulsar.Client('pulsar://localhost:6650')
consumer = client.subscribe('persistent://public/default/my-topic',
subscription_name='my-subscription',
consumer_type=pulsar.ConsumerType.Exclusive)
# Assume you have a message ID you want to jump to
# In a real scenario, you'd get this from a previous read or an index
message_id_to_seek = pulsar.MessageId(ledger_id=123, entry_id=456, partition=0)
# Seek to the message ID
consumer.seek(message_id_to_seek)
print(f"Seeked to message ID: {message_id_to_seek}")
# Now, when you receive messages, they will start from or after this point
msg = consumer.receive()
print(f"Received message: {msg.data()} from message ID: {msg.message_id()}")
client.close()
And here’s how you’d do it with a timestamp:
import pulsar
import time
client = pulsar.Client('pulsar://localhost:6650')
consumer = client.subscribe('persistent://public/default/my-topic',
subscription_name='my-subscription',
consumer_type=pulsar.ConsumerType.Exclusive)
# Seek to a specific timestamp (e.g., 1 hour ago)
one_hour_ago_ms = int((time.time() - 3600) * 1000)
consumer.seek_by_timestamp(one_hour_ago_ms)
print(f"Seeked to timestamp: {one_hour_ago_ms} ms")
# Receive messages starting from that timestamp
msg = consumer.receive()
print(f"Received message: {msg.data()} from message ID: {msg.message_id()} at timestamp: {msg.publish_time()}")
client.close()
The Mental Model: Log-Based Storage and Cursor Management
Pulsar’s core is a distributed log. Each topic is essentially a sequence of entries appended to ledgers. When you consume, you’re moving a cursor through this log. seek doesn’t re-process or re-store data; it simply repositions your consumer’s cursor.
When you seek to a MessageId, Pulsar’s broker finds the ledger and entry corresponding to that ID and tells your consumer to start reading from there. For seek_by_timestamp, the broker efficiently finds the first entry whose publish timestamp is greater than or equal to the requested timestamp within the topic’s ledgers. This is remarkably fast because Pulsar maintains metadata (like index files) that allow it to quickly locate entries by ID or time without scanning the entire log.
The MessageId itself is a composite key: ledger_id:entry_id:partition. The ledger_id identifies the specific log segment, entry_id is the sequential position within that ledger, and partition indicates which partition of the topic the message belongs to. When you receive a message, its message_id is what you use to track your progress.
The core problem seek solves is enabling "replayability" and "recovery" without complex application-level logic. Instead of building a separate indexing service or re-ingesting data to find a specific point, you can use seek to reposition your existing consumer. This is invaluable for debugging, reprocessing data after a bug fix, or catching up on events from a specific historical moment.
The distinction between seek and seek_by_timestamp is crucial. seek(message_id) is precise – it targets an exact message. seek_by_timestamp(timestamp) is an upper bound; it finds the earliest message whose publish time is at or after your specified timestamp. If no message matches precisely, it will jump to the next available message, potentially after your target time.
The true power of seek is in how it interacts with Pulsar’s durable, append-only log. Unlike traditional message queues where seeking might imply replaying from the beginning and filtering, Pulsar’s architecture allows this operation to be a metadata lookup and cursor adjustment. This means low latency and efficient use of resources. It’s not about finding data; it’s about telling the broker where to start serving data from your current consumer’s perspective.
The next logical step after mastering seek is understanding how to use it in conjunction with Pulsar’s tiered storage, allowing you to seek into historical data that has been offloaded to cheaper storage layers.