tencent cloud

Overview

When the client initiates an access request to an address, it will query whether the local DNS cache has relevant records, and if so, it will directly access the corresponding IP; otherwise, it will delegate global query to the recursive server.

As DNS resolution uses the UDP protocol for communication, it is greatly affected by the network environment and may have a delay of several seconds in extreme conditions. In SCF use cases, the domain resolution delay may cause function execution failures due to timeout and affect the normal business logic. When a function is invoked frequently, the DNS server's resolution frequency may exceed the limit, which also will cause function execution failures.

SCF offers DNS caching configuration to solve the above problems. DNS caching can improve the domain resolution efficiency and increase the domain resolution success rate by mitigating network jitters.

Use Cases

This feature is applicable to scenarios where an address is requested in the function code and the function is invoked frequently.

Directions

As code deployment-based event-triggered functions, HTTP-triggered functions, and image deployment-based functions have different implementation mechanisms, you should enable DNS caching in the corresponding steps:

Code deployment-based event-triggered function

1. Log in to the Serverless console and select the target function to enter the function details page.
2. On the function configuration page, click Edit in the top-right corner and select Enable DNS Caching.
3. Click Save.

HTTP-triggered function

1. Add the following command to the HTTP-triggered function's bootstrap file scf_bootstrap to start the nscd process and enable DNS caching:

   /var/lang/bin/nscd -f /var/lang/conf/nscd.conf
   

2. Deploy the updated scf_bootstrap and the function code together in the cloud. Then, DNS caching will be enabled for new invocations.

Image deployment-based function

1. Install nscd during image creation. Taking CentOS as an example, you can run the following command to install nscd:

   yum install nscd -y
   

2. Update the default /etc/nscd.conf file with the following content:

   #
   # /etc/nscd.conf
   #
   # An example Name Service Cache config file. This file is needed by nscd.
   #
   # WARNING: Running nscd with a secondary caching service like sssd may lead to
   # unexpected behaviour, especially with how long entries are cached.
   #
   # Legal entries are:
   #
   # logfile <file>
   # debug-level <level>
   # threads <initial #threads to use>
   # max-threads <maximum #threads to use>
   # server-user <user to run server as instead of root>
   # server-user is ignored if nscd is started with -S parameters
   # stat-user <user who is allowed to request statistics>
   # reload-count unlimited|<number>
   # paranoia <yes|no>
   # restart-interval <time in seconds>
   #
   # enable-cache <service> <yes|no>
   # positive-time-to-live <service> <time in seconds>
   # negative-time-to-live <service> <time in seconds>
   # suggested-size <service> <prime number>
   # check-files <service> <yes|no>
   # persistent <service> <yes|no>
   # shared <service> <yes|no>
   # NOTE: Setting 'shared' to a value of 'yes' will accelerate the lookup,
   # but those lookups will not be counted as cache hits
   # i.e. 'nscd -g' may show '0%'.
   # max-db-size <service> <number bytes>
   # auto-propagate <service> <yes|no>
   #
   # Currently supported cache names (services): passwd, group, hosts, services
   #
   # logfile /var/log/nscd.log
   # threads 4
   # max-threads 32
   server-user root
   # stat-user somebody
   debug-level 0
   reload-count 2
   paranoia no
   # restart-interval 3600
   enable-cache passwd no
   positive-time-to-live passwd 600
   negative-time-to-live passwd 20
   suggested-size passwd 211
   check-files passwd yes
   persistent passwd yes
   shared passwd yes
   max-db-size passwd 33554432
   auto-propagate passwd yes
   enable-cache group no
   positive-time-to-live group 3600
   negative-time-to-live group 60
   suggested-size group 211
   check-files group yes
   persistent group yes
   shared group yes
   max-db-size group 33554432
   auto-propagate group yes
   enable-cache hosts yes
   positive-time-to-live hosts 300
   negative-time-to-live hosts 0
   suggested-size hosts 211
   check-files hosts no
   persistent hosts no
   shared hosts yes
   max-db-size hosts 8388608
   enable-cache services no
   positive-time-to-live services 600
   negative-time-to-live services 3
   suggested-size services 211
   check-files services yes
   persistent services yes
   shared services yes
   max-db-size services 33554432
   enable-cache netgroup no
   positive-time-to-live n etgroup 28800
   negative-time-to-live netgroup 20
   suggested-size netgroup 211
   check-files netgroup yes
   persistent netgroup yes
   shared netgroup yes
   max-db-size netgroup 33554432
   

3. Add the following command to the bootstrap file scf_bootstrap to start nscd process and enable DNS caching.
   Taking CentOS as an example, add the following command to the bootstrap file:

   ${PATH}/nscd -f /etc/nscd.conf 
   

Note：

${PATH} is the absolute path where nscd is installed.