Loops for polling

You can use Postman Flows to build a “while do” loop that polls an API for new values until a condition is satisfied.

Like the previous topic, Loops for pagination, this topic describes a design pattern that allows you break out of a loop:

• Use an HTTP Request block to obtain a value from the Postman Echo API – that is, to poll the API.

• Use a Condition block to test whether the value satisfies a condition. If the test returns false, iterate: return to the beginning of the loop and poll the API again. If the test returns true, break out of the loop and stop.

As long as the loop keeps polling, you know that the condition hasn’t been satisfied yet. In a sense, this enables your flow to manipulate state across multiple iterations of the loop.

Build a loop for polling

Here’s how the finished loop will look:

Before building the flow, create an HTTP GET request in a collection of your choice. Configure the request with the following URL:

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https://postman-echo.com/get?randomNumber={{$randomInt}}

Rename the request “Random number generator” and click Save.

Create a new flow.

Construct the loop

In this section you’ll add blocks to build a loop, including a Condition block to determine whether to continue iterating or stop.

1. Create an HTTP Request block, choosing the “Random number generator” HTTP request you created above. This renames the block as Random number generator.

2. Connect the output of the Start block to the Send input of the Random number generator block.

3. Connect a Condition block to the Random number generator block’s Success output and rename it “Condition: the number must be divisible by 5”. In the value1 input’s inline Select block, enter the path body.args.randomNumber.

4. Paste the following expression into the TypeScript window:

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   value1 % 5 === 0 ? true : false

   When value1 is divisible by 5, the expression returns true and the loop terminates. Otherwise the expression returns false and the flow iterates again, polling the API for a new value.

5. Connect a Delay block to the Default output of the Condition: the number must be divisible by 5 block and set its value to 1000 milliseconds.

6. Connect the Delay block’s output to the Send input of the Random number generator block. Note that this connection loops back to the beginning, making the flow a true loop in form, not only in function.

Add blocks to display status

The blocks you add in this section will enable you to follow the loop’s action throughout its lifecycle.

1. Connect a Template block to the Condition 1 output of the Random number generator block and rename it Met the condition, stop. Rename key as inputInt. In the inline Select block, click Enter path... and select value1. Paste the following expression into the text window:

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   {{inputInt}} meets the condition:
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   it's divisible by 5.
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   We can stop now!

2. Connect a Template block to the Default output of the Random number generator block and rename it Didn’t meet the condition, keep polling. Rename key as inputInt. In the inline Select block, click Enter path... and select value1. Paste the following expression into the text window:

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   {{inputInt}} is not divisible by 5.
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   Keep polling!

3. Connect a Display block to both the Met the condition, stop and the Didn’t meet the condition, keep polling block. From Additional Settings, select Text size > Large.

Run the flow. You’ll see each block activate in sequence and the loop iterate until the API it’s polling produces a number that’s divisible by 5. At that point, the condition you specified is satisfied and the loop terminates.