Open file cache in Nginx - Step-by-Step Execution

Process Flow - Open file cache

Request for file

↓

Check open file cache

↓

Serve file

↓

Return response

↓

Serve file

↓

Return response

When nginx receives a file request, it first checks the open file cache. If the file is cached (hit), it serves it directly. If not (miss), it opens the file from disk, adds it to the cache, then serves it.

Execution Sample

Nginx

open_file_cache max=1000 inactive=20s;
open_file_cache_valid 30s;
open_file_cache_min_uses 2;

location /images/ {
}

This config sets up nginx to cache open files with limits and validity times, improving file serving performance.

Process Table

Step	Action	Cache State	File Opened	Response
1	Request file /images/logo.png	Empty	No	No response yet
2	Check cache for /images/logo.png	Empty	No	Cache miss
3	Open /images/logo.png from disk	Empty	Yes	No response yet
4	Add /images/logo.png to cache	Contains /images/logo.png	Yes	No response yet
5	Serve /images/logo.png from cache	Contains /images/logo.png	Yes	File content sent
6	Request file /images/logo.png again	Contains /images/logo.png	No	No response yet
7	Check cache for /images/logo.png	Contains /images/logo.png	No	Cache hit
8	Serve /images/logo.png from cache	Contains /images/logo.png	No	File content sent
9	Request file /images/banner.png	Contains /images/logo.png	No	No response yet
10	Check cache for /images/banner.png	Contains /images/logo.png	No	Cache miss
11	Open /images/banner.png from disk	Contains /images/logo.png	Yes	No response yet
12	Add /images/banner.png to cache	Contains /images/logo.png, /images/banner.png	Yes	No response yet
13	Serve /images/banner.png from cache	Contains /images/logo.png, /images/banner.png	Yes	File content sent
14	Cache entry /images/logo.png inactive for 21s	Contains /images/logo.png, /images/banner.png	Yes	No response yet
15	Remove /images/logo.png from cache due to inactivity	Contains /images/banner.png	No	No response yet
16	Request file /images/logo.png	Contains /images/banner.png	No	No response yet
17	Check cache for /images/logo.png	Contains /images/banner.png	No	Cache miss
18	Open /images/logo.png from disk	Contains /images/banner.png	Yes	No response yet
19	Add /images/logo.png to cache	Contains /images/banner.png, /images/logo.png	Yes	No response yet
20	Serve /images/logo.png from cache	Contains /images/banner.png, /images/logo.png	Yes	File content sent

💡 Execution stops after serving files and cache updates; cache entries removed after inactivity timeout.

Status Tracker

Variable	Start	After Step 4	After Step 8	After Step 12	After Step 15	After Step 20
open_file_cache	empty	/images/logo.png	/images/logo.png	/images/logo.png, /images/banner.png	/images/banner.png	/images/banner.png, /images/logo.png
file_opened	No	Yes	No	Yes	No	Yes
response_sent	No	No	Yes	No	No	Yes

Key Moments - 3 Insights

Why does nginx open the file from disk even though it was requested before?

Because the file was removed from the cache after being inactive for longer than the inactive timeout (step 15), so nginx must open it again (step 18).

What happens when a file is requested and found in the open file cache?

Nginx serves the file directly from the cache without opening it again on disk, as shown in steps 6-8.

How does nginx decide to remove a file from the open file cache?

If a cached file is inactive longer than the configured inactive time (20s here), nginx removes it from the cache (step 15).

Visual Quiz - 3 Questions

Look at the execution table, at which step does nginx add /images/banner.png to the cache?

A Step 10

B Step 12

C Step 14

D Step 16

💡 Hint

Check the 'Action' column for 'Add /images/banner.png to cache' in the execution table.

According to the variable tracker, what is the state of open_file_cache after step 15?

A Contains /images/logo.png and /images/banner.png

B Empty

C Contains /images/banner.png only

D Contains /images/logo.png only

💡 Hint

Look at the 'open_file_cache' row under 'After Step 15' in the variable tracker.

If the inactive time was set to 60s instead of 20s, what would change in the execution table?

A Step 15 (removal of /images/logo.png) would not occur

B Cache misses would increase

C Files would never be added to cache

D Files would be opened from disk every time

💡 Hint

Consider the cache removal step based on inactivity timeout in the execution table.

Concept Snapshot

open_file_cache caches open file descriptors to speed up file serving.
Configure max entries, inactive timeout, and validity.
Cache hit serves file directly; miss opens file and caches it.
Inactive files removed after timeout to save resources.
Improves performance by reducing disk opens.

Full Transcript

This visual execution shows how nginx handles open file caching. When a file is requested, nginx checks if it is in the open file cache. If found (cache hit), nginx serves it directly without opening the file again on disk. If not found (cache miss), nginx opens the file from disk, adds it to the cache, then serves it. Cached files are removed after being inactive longer than the configured inactive timeout. The execution table traces requests, cache hits and misses, file openings, and cache updates step-by-step. The variable tracker shows how the cache contents and file open states change over time. Key moments clarify why files are reopened after removal and how cache hits speed up serving. The quiz tests understanding of cache addition, removal, and configuration effects. This helps beginners see exactly how open file caching works in nginx to improve performance.