HTTP Error 503

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By Sina Nasiri


HTTP status codes are notifications received in response to a client’s report to a server. For instance, when your client (your web browser) attempts to link to the site, you try to connect to whichever the server is. The server displays various responses depending on how the query is managed. Redirects, database errors, client errors, and other specific results are instances of such errors. HTTP error codes are not components of web pages; rather, they are server responses on how the request is handled.

That being said, it is important to note that not all the http status codes specify errors. For instance, Some simply remind users that a website has been relocated, either indefinitely or temporarily. However, if you encounter errors, the HTTP error codes shown will assist you in determining the source of the problem.


Let us look at some other common http codes. Http codes are not just limited to error codes, and also response HTTP response status codes show whether or not a particular HTTP request was approached. The replies are divided into different categories:

  • Client errors
  • Redirects
  • Server Errors
  • Successful Responses
  • Informational Responses

When you get a response that isn’t on this page, it’s a non-standard response, probably tailored to the server’s program.

Client Error Responses

The following are the error responses under the following heading

  • 400 Bad Request
  • 401 Unauthorized
  • 402 Payment Required
  • 403 Forbidden
  • 404 Not Found
  • 405 Method Not Allowed and more to go

Redirection Messages

The following are the error responses under the following heading

  • 300 Multiple Choice
  • 301 Moved Permanently
  • 302 Found
  • 303 See Other
  • 304 Not Modified
  • 305 Use Proxy and more to go

Server Error Responses

The following are the error responses under the following heading

  • 500 Internal Server Error
  • 501 Not Implemented
  • 502 Bad Gateway
  • 503 Service Unavailable
  • 504 Gateway Timeout
  • 505 HTTP Version Not Supported and more to go

Browser Compatibility

The following are the error responses under the following heading

  • Chrome – Yes
  • Firefox – Yes
  • Internet Explorer- Yes
  • Opera- Yes
  • Safari- Yes and more to go


The 503 Service of the HyperText Transfer Protocol The unavailable error response code of the server shows that the server is not ready to process the application. Common triggers include a server down or crowded with repairs. The HTTP header of Retry-after should include, if possible, the approximate time for service recovery, which should be used on provisional terms. Note: a user-friendly website should be submitted along with this answer to clarify the issue. The header associated with the caching should be looked at since 503 is always a transient state, and replies should not be cached.




The HTTP status code 503 makes it impossible for the server to accommodate incoming requests. This mistake usually happens due to the overtime or temporary repair of the server.

After this forked processing is done after completing the API requests, it can be turned on or off, which means the API user may or may not get the intended result immediately but might receive a different URL that might get the result.

Hence picking the right http code is a little better than going or a completely random code.

Possible causes for the Error 503 are

  • OVERLOADED SERVER – The description of the error is “The backend server is either overwhelmed or unable to accommodate new demands from the incoming customer”, and The edge public and private users can operate the troubleshooting steps.
  • SERVER UNDER MAINTENANCE – The description of the error is “ The backend server could be momentarily maintained”.


You can use one of three ways to diagnose the error:

  • Tool for tracking- activate the trace session and find the API call unavailable to replicate it – 503 Service.
  • Choose one of the missing applications and check the trace.
  • Navigate through different trace phases to see where the fault happened.
  • When the error 503 is returned to a target server as a response to the error, the source of the error 503 is the target server.
  • This is a screenshot showing a 503 Service unavailable reaction from a target server.
  • Click on the response from the goal server process to view whether they provide valuable details through the Answer headers and Response Content sections:
  • The answer headers may include the server header indicating the source of the error reply.
  • Additional detail on why the target server submitted a 503 response code could be included in the response material.

  • Logs for Nginx- The Nginx access logs will also be used to decide whether you have received a 503 status code from the backend server. This is especially helpful when the problem has happened in the past or if the problem is sporadic and you cannot track it in a user interface. Use this information in Nginx access logs to decide by following steps:
    • Check access logs
    • Search for any 503 API proxy error at a particular time (if there was a problem) or any request still failed with 503. Search for any 503 error.
    • If there are 503 errors, verify that the error comes from the server backend. The error of 503 is from the Backend server if the values of X-Apigee-default-source and X-Apigee-default-code correspond to those of the values. { Values- X-Apigee-default-source- Target; X-Apigee-default-code- messaging.adaptors.http.flow.ErrorResponseCode}
    • Check the specific API proxy to ensure it is used, i.e., if the target server/target endpoint does not call up another proxy in Apigee. You need to repeat all the steps above with the chained proxy if you are using proxy chaining before determining the cause of the 503 services not available. In these situations, 503 unavailable services can also occur in other chained proxies, which you may diagnose with this playbook.
    • If you check that you do not use proxy chaining and your backend server contains the 503 bugs, go-to Resolution.
  • Backend node direct call – You should call the server directly to make sure you get the same 503 services unavailable response as you received when you received the request from Apigee Edge.
  • Ensure that you have all the headers, query parameters, and credentials needed as part of your request to transfer to the backend server.
  • You can use the Curl command, Postman, or some other REST client to invoke the Backend Server API directly if the backend service is open to the public.
  • You can use the command curl, Postman, or any other REST client and invoke the backend server API directly via the Message Processor if the backend server is accessible only from the Message Processors.


Let us further see how to fix a 503 Not Modified Status Code stepwise :

You should do the following to fix the problem by checking that the 503 error comes from the backend server:

  • If the backend server’s maintenance triggers the problem, you should put the backend server online after maintenance.
  • If the backend server’s overload triggers the problem, correct the issue when you have backend server access. Otherwise, the server backend team will have to work to correct the issue.


API Monitoring allows you to easily isolate trouble areas to detect and diagnose the fault, efficiency, and latency problems, such as developer applications, API proxies, backend goals, and API surveillance. For instance, when you message a number, you may want to set up an alarm. Faults with adaptors.http.flow.ErrorResponseCode surpasses a certain threshold.

Especially, If those errors come from the backend server or go incorrectly, another proxy called part of a proxy chaining, the API Monitoring lists 503 errors in the error code messages.adaptors.http.flow.ErrorResponseCode.

You will like the warning for the 503 status code to be set up using fault code as messaging.adaptors.http.flow.ErrorResponseCode whether a 503 error message from the backend server will be alerted.


There is a list of the common http errors that occur; let us see what they are and what they mean:


The error 403 shares similarities with the 401 error but the one minute difference between forbidden and unauthorized is what the words mean. In this case, you cannot log in even if you want to because you do not have the opportunity. This can happen when you are trying to log in to a university Knowledge portal, and you don’t go to the university.


I, for one, have seen such hilarious memes on this one, but again it is a meme because it is again one of the most common http errors that occur; this happens when you try to access a page whose URL is broken, or it does not exist.

Sometimes the common reason is us mistyping a URL, or the website is moved to another link or deleted completely.


This one again is a no-brainer and means what the name is, and this happens when you go to an application using the browser and request that the specific website cannot perform. This error is probably just the web server telling you the application or the web browser you are using has been corrupted or accessed incorrectly.


We previously discussed the difference between 403 and 401, but now let us see what 401 is. Like it is obvious from the name, this error is prone to arise or occur when the user tries to access a website with recruited access and is not authorized to do so and failed login attempts.


The description of why this error occurs is again a no brainer as the name explains it all if there is an error in the internal control of a server like an overload or something that makes it impossible for the server to process your request, that is when you know there is an http 500 error.


The 502 status code, often recognized as a Bad Gateway error, implies that the server is a gateway or proxy server and is not obtaining a legitimate response from the backend servers that should satisfy the order.

  • HTTP ERROR 305

Now, let us discuss more on the Http error 305 specifically. This http error is specific to the Redirection Messages. The Error 305 is called Use Proxy. In a previous version of the HTTP specification, this term was used to mean that a requested response could be reached via a proxy. It has been deprecated due to security issues over proxy in-band setup.

According to the http error, 305 is the requested resource that should be accessed through the proxy specified in the Location field. The Location field contains the proxy’s URI. This single request is required to be repeated by the receiver via the proxy. Origin servers MUST ONLY produce 305 replies. The HTTP Status Code 305 indicates that the client’s accessed resource is only accessible via a proxy, and the address for the proxy is given in the server’s response. Described in an earlier version of this requirement which is now discontinued security breaches around proxy in-band configuration.

However, there are a few ways that can be used to troubleshoot the common HTTP errors. Down below are a few things you can try to fix the problem before looking for a full-blown solution for it:

  1. When reviewing a web server with a web browser, restart the browser after making server updates.
  2. Assess the service logs for more information about how the server is processing the requests.
  3. Keep in mind that HTTP status code specifications are part of a protocol that the serving requests program follows. This ensures that the final status code returned is determined by how the server program treats a specific error–however, this model should normally show you towards the correct path.
  4. The solution can be error specific most of the time, and identifying the error is half the solution; for instance, when there is an error 301, we know the website has been moved permanently. We can try looking for it by entering the keywords and finding something similar or that even.


You must have a solid foundation for troubleshooting issues with your web servers or apps now that you are acquainted with one of the most prevalent HTTP error codes and common solutions to such codes. This article was an overview of how to troubleshoot the http errors and find out the types of errors. More specifically, it dealt with the ERROR 503 Service Not Available, and what causes it and what can fix it. I hope you find the article insightful and that it answers your basic queries about what HTTP errors are and what can be done to get through and fix them.

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