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.

Practice

1. What is the main purpose of the open_file_cache directive in nginx?

easy

A. To limit the number of open connections to clients

B. To store file information in memory to speed up file access

C. To cache HTTP responses from upstream servers

D. To compress files before sending them to clients

Solution

1. Step 1: Understand the role of open_file_cache

   The open_file_cache directive caches file metadata like file descriptors and attributes.

2. Step 2: Identify its effect on performance

   By caching file info, nginx reduces disk access and speeds up serving static files.

3. Final Answer:

   To store file information in memory to speed up file access -> Option B

4. Quick Check:

   open_file_cache purpose = cache file info [OK]

Hint: Remember: open_file_cache speeds file access by caching file info [OK]

Common Mistakes:

• Confusing open_file_cache with HTTP response caching
• Thinking it limits client connections
• Assuming it compresses files

2. Which of the following is the correct syntax to enable open file cache with 1000 max cached files and 60 seconds inactive timeout?

easy

A. open_file_cache max=1000 inactive=60;

B. open_file_cache 1000 60s;

C. open_file_cache max=1000 inactive=60s;

D. open_file_cache max=1000 inactive=60sec;

Solution

1. Step 1: Recall correct open_file_cache syntax

   The syntax requires named parameters like max= and inactive= with time units.

2. Step 2: Check each option for correctness

   open_file_cache max=1000 inactive=60s; uses correct parameter names and time unit s for seconds.

3. Final Answer:

   open_file_cache max=1000 inactive=60s; -> Option C

4. Quick Check:

   Use named params with units for open_file_cache [OK]

Hint: Use named parameters with units like 'inactive=60s' [OK]

Common Mistakes:

• Omitting parameter names
• Missing time units for inactive
• Using incorrect time unit abbreviations

3. Given this nginx config snippet:

open_file_cache max=2 inactive=30s;
open_file_cache_valid 10s;

What happens when nginx tries to serve 3 different files within 10 seconds?

medium

A. All 3 files are cached because open_file_cache_valid is 10s

B. All files are cached but expire after 30 seconds

C. No files are cached because max is less than 3

D. Only 2 files are cached; the third file info is not cached

Solution

1. Step 1: Understand max parameter effect

   The max=2 limits cache to 2 file entries only.

2. Step 2: Analyze caching of 3 files

   When 3 files are requested, only 2 can be cached; the third is not cached.

3. Final Answer:

   Only 2 files are cached; the third file info is not cached -> Option D

4. Quick Check:

   max=2 limits cache size to 2 files [OK]

Hint: max limits cached files count; extras are not cached [OK]

Common Mistakes:

• Assuming all files cache regardless of max
• Confusing open_file_cache_valid with max
• Thinking inactive controls cache size

4. You configured open_file_cache max=100 inactive=60s; but notice no caching effect. What is a likely cause?

medium

A. You forgot to enable open_file_cache_valid directive

B. The max value is too high to cache files

C. The inactive time is too short to cache files

D. open_file_cache only works with dynamic content

Solution

1. Step 1: Check required directives for caching

   Besides open_file_cache, open_file_cache_valid must be set to define cache validity.

2. Step 2: Identify missing directive effect

   Without open_file_cache_valid, cached entries expire immediately, so no caching effect.

3. Final Answer:

   You forgot to enable open_file_cache_valid directive -> Option A

4. Quick Check:

   open_file_cache_valid enables cache duration [OK]

Hint: Always set open_file_cache_valid to activate caching [OK]

Common Mistakes:

• Ignoring open_file_cache_valid directive
• Misunderstanding max and inactive roles
• Thinking open_file_cache works only for dynamic content

5. You want to optimize nginx for a high-traffic static site. Which open_file_cache settings best improve performance while avoiding stale data?

hard

A. open_file_cache max=1000 inactive=60s; open_file_cache_valid 30s;

B. open_file_cache max=10 inactive=300s; open_file_cache_valid 300s;

C. open_file_cache max=5000 inactive=5s; open_file_cache_valid 1s;

D. open_file_cache max=100 inactive=3600s; open_file_cache_valid 3600s;

Solution

1. Step 1: Balance cache size and freshness

   High-traffic static sites benefit from a large cache (max=1000) and moderate inactive timeout (60s) to keep data fresh.

2. Step 2: Set cache validity to avoid stale data

   open_file_cache_valid 30s ensures cached info refreshes often enough to prevent stale data.

3. Final Answer:

   open_file_cache max=1000 inactive=60s; open_file_cache_valid 30s; -> Option A

4. Quick Check:

   Large max + moderate inactive + short valid = best performance [OK]

Hint: Use large max, moderate inactive, short valid for fresh cache [OK]

Common Mistakes:

• Setting max too low for traffic
• Using too long inactive or valid causing stale cache
• Setting valid too short causing overhead