Pull vs push subscriptions in GCP - Visual Side-by-Side Comparison

Process Flow - Pull vs push subscriptions

Message Published

↓

Message Delivered

↓

Message Published

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Client Pulls Message

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Message Delivered

Messages are sent to subscribers either by pushing to an endpoint automatically or by subscribers pulling messages when ready.

Execution Sample

GCP

1. Publish message to topic
2a. Push subscription sends message to endpoint
2b. Pull subscription waits for client to request
3. Client receives message

Shows the two ways messages reach subscribers: push sends automatically, pull waits for client request.

Process Table

Step	Action	Subscription Type	Message State	Result
1	Message published to topic	N/A	Message in topic	Message ready for delivery
2a	Push subscription receives message	Push	Message in subscription	Message sent to endpoint URL
3a	Endpoint acknowledges message	Push	Message delivered	Message removed from subscription
2b	Pull subscription receives message	Pull	Message in subscription	Message waits for client pull
3b	Client pulls message	Pull	Message delivered	Message acknowledged and removed
4	No more messages	Both	No messages	Waiting for new messages

💡 No messages left in subscription; system waits for new messages.

Status Tracker

Variable	Start	After Step 1	After Step 2a	After Step 3a	After Step 2b	After Step 3b	Final
Message State	None	In Topic	In Push Subscription	Delivered and Removed	In Pull Subscription	Delivered and Removed	None

Key Moments - 3 Insights

Why does the pull subscription keep the message until the client pulls it?

Because pull subscriptions wait for the client to request messages, the message stays in the subscription until pulled, as shown in step 2b and 3b in the execution_table.

How does the push subscription deliver messages automatically?

Push subscriptions send messages immediately to a configured endpoint URL without waiting, as seen in steps 2a and 3a in the execution_table.

What happens if the endpoint in push subscription does not acknowledge the message?

The message remains undelivered and may be retried until acknowledged, but this is outside the current table scope; the key is that acknowledgment removes the message as in step 3a.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the pull subscription deliver the message to the client?

A Step 3b

B Step 1

C Step 2a

D Step 3a

💡 Hint

Check the 'Subscription Type' and 'Result' columns for pull subscription delivery.

According to variable_tracker, what is the message state after step 3a?

A In Topic

B In Pull Subscription

C Delivered and Removed

D None

💡 Hint

Look at the 'After Step 3a' column for 'Message State'.

If the client never pulls messages in a pull subscription, what happens to the message?

A It is automatically sent to an endpoint

B It stays in the subscription waiting

C It is deleted immediately

D It is sent back to the topic

💡 Hint

Refer to step 2b and 3b in the execution_table and variable_tracker.

Concept Snapshot

Pull vs Push Subscriptions:
- Push: Messages sent automatically to endpoint URL.
- Pull: Client requests messages when ready.
- Push requires endpoint acknowledgment.
- Pull holds messages until client pulls.
- Choose based on delivery control needs.

Full Transcript

This visual execution shows how messages flow in Google Cloud Pub/Sub using pull and push subscriptions. When a message is published, push subscriptions immediately send it to a configured endpoint, which must acknowledge receipt to remove the message. Pull subscriptions keep the message until a client explicitly requests it, giving the client control over when to receive messages. The execution table traces each step, showing message states and actions. Variable tracking highlights how message state changes from being in the topic, to subscription, to delivered and removed. Key moments clarify common confusions about message delivery timing and acknowledgment. The quiz tests understanding by referencing specific steps and states in the tables.