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util: Extract helper function for backoff of retries on error response.

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Extracts message_fetch and get_events algorithms for calculating the
exponential backoff into a shared helper function.

These match the algorithm that were designed for the Python API, except
that we use a ratio of 2 rather than sqrt(2) in the message_fetch code
path.
This commit is contained in:
Lauryn Menard
2025-01-29 21:00:36 +01:00
committed by Tim Abbott
parent 79224b0149
commit 078ce89380
4 changed files with 84 additions and 40 deletions
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24
web/src/message_fetch.ts
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@@ -28,6 +28,7 @@ import {narrow_term_schema} from "./state_data.ts";
import * as stream_data from "./stream_data.ts";
import * as stream_list from "./stream_list.ts";
import * as ui_report from "./ui_report.ts";
import * as util from "./util.ts";
const response_schema = z.object({
anchor: z.number(),
@@ -444,28 +445,7 @@ export function load_messages(opts: MessageFetchOptions, attempt = 1): void {
ui_report.show_error($("#connection-error"));
// We need to respect the server's rate-limiting headers, but beyond
// that, we also want to avoid contributing to a thundering herd if
// the server is giving us 500s/502s.
//
// So we do the maximum of the retry-after header and an exponential
// backoff with ratio 2 and half jitter. Starts at 1-2s and ends at
// 16-32s after 5 failures.
const backoff_scale = Math.min(2 ** attempt, 32);
const backoff_delay_secs = ((1 + Math.random()) / 2) * backoff_scale;
let rate_limit_delay_secs = 0;
const rate_limited_error_schema = z.object({
"retry-after": z.number(),
code: z.literal("RATE_LIMIT_HIT"),
});
const parsed = rate_limited_error_schema.safeParse(xhr.responseJSON);
if (xhr.status === 429 && parsed?.success && parsed?.data) {
// Add a bit of jitter to the required delay suggested by the
// server, because we may be racing with other copies of the web
// app.
rate_limit_delay_secs = parsed.data["retry-after"] + Math.random() * 0.5;
}
const delay_secs = Math.max(backoff_delay_secs, rate_limit_delay_secs);
const delay_secs = util.get_retry_backoff_seconds(xhr, attempt, true);
setTimeout(() => {
load_messages(opts, attempt + 1);
}, delay_secs * 1000);